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Add Prefix Sum and Hash Map / Set problems with solution
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‎README.md

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## Problems and Solutions
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| Category | Problem | Solution | Difficulty |
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|----------------|---------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------|------------|
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| Array / String | [Merge Strings Alternately](https://leetcode.com/problems/merge-strings-alternately/) | [MergeStringsAlternately.java](src/array_string/MergeStringsAlternately.java) | Easy |
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| Array / String | [Greatest Common Divisor of Strings](https://leetcode.com/problems/greatest-common-divisor-of-strings/) | [GreatestCommonDivisorOfStrings.java](src/array_string/GreatestCommonDivisorOfStrings.java) | Easy |
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| Array / String | [Kids With the Greatest Number of Candies](https://leetcode.com/problems/kids-with-the-greatest-number-of-candies/) | [GreatestNumberOfCandies.java](src/array_string/GreatestNumberOfCandies.java) | Easy |
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| Array / String | [Can Place Flowers](https://leetcode.com/problems/can-place-flowers/) | [CanPlaceFlowers.java](src/array_string/CanPlaceFlowers.java) | Easy |
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| Array / String | [Reverse Vowels of a String](https://leetcode.com/problems/reverse-vowels-of-a-string/) | [ReverseVowelsOfString.java](src/array_string/ReverseVowelsOfString.java) | Easy |
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| Array / String | [Reverse Words in a String](https://leetcode.com/problems/reverse-words-in-a-string/) | [ReverseWordsInString.java](src/array_string/ReverseWordsInString.java) | Medium |
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| Array / String | [Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/) | [ProductExceptSelf.java](src/array_string/ProductExceptSelf.java) | Medium |
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| Array / String | [Increasing Triplet Subsequence](https://leetcode.com/problems/increasing-triplet-subsequence/) | [IncreasingTriplet.java](src/array_string/IncreasingTriplet.java) | Medium |
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| Array / String | [String Compression](https://leetcode.com/problems/string-compression/) | [StringCompressor.java](src/array_string/StringCompressor.java) | Medium |
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| Two Pointers | [Move Zeroes](https://leetcode.com/problems/move-zeroes/) | [ZeroMover.java](src/two_pointers/ZeroMover.java) | Easy |
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| Two Pointers | [Is Subsequence](https://leetcode.com/problems/is-subsequence/) | [SubsequenceChecker.java](src/two_pointers/SubsequenceChecker.java) | Easy |
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| Two Pointers | [Container With Most Water](https://leetcode.com/problems/container-with-most-water/) | [WaterContainer.java](src/two_pointers/WaterContainer.java) | Medium |
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| Two Pointers | [Max Number of K-Sum Pairs](https://leetcode.com/problems/max-number-of-k-sum-pairs/) | [MaxOperationsCounter.java](src/two_pointers/MaxOperationsCounter.java) | Medium |
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| ... | ... | ... | ... |
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| Trie | [Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree) | [PrefixTrie.java](src/trie/PrefixTrie.java) | Medium |
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| Trie | [Search Suggestions System](https://leetcode.com/problems/search-suggestions-system) | [SearchSuggestionsSystem.java](src/trie/SearchSuggestionsSystem.java) | Medium |
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| ... | ... | ... | ... |
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| Category | Problem | Solution | Difficulty |
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|----------------|---------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------|------------|
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| Array / String | [Merge Strings Alternately](https://leetcode.com/problems/merge-strings-alternately/) | [MergeStringsAlternately.java](src/array_string/MergeStringsAlternately.java) | Easy |
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| Array / String | [Greatest Common Divisor of Strings](https://leetcode.com/problems/greatest-common-divisor-of-strings/) | [GreatestCommonDivisorOfStrings.java](src/array_string/GreatestCommonDivisorOfStrings.java) | Easy |
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| Array / String | [Kids With the Greatest Number of Candies](https://leetcode.com/problems/kids-with-the-greatest-number-of-candies/) | [GreatestNumberOfCandies.java](src/array_string/GreatestNumberOfCandies.java) | Easy |
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| Array / String | [Can Place Flowers](https://leetcode.com/problems/can-place-flowers/) | [CanPlaceFlowers.java](src/array_string/CanPlaceFlowers.java) | Easy |
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| Array / String | [Reverse Vowels of a String](https://leetcode.com/problems/reverse-vowels-of-a-string/) | [ReverseVowelsOfString.java](src/array_string/ReverseVowelsOfString.java) | Easy |
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| Array / String | [Reverse Words in a String](https://leetcode.com/problems/reverse-words-in-a-string/) | [ReverseWordsInString.java](src/array_string/ReverseWordsInString.java) | Medium |
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| Array / String | [Product of Array Except Self](https://leetcode.com/problems/product-of-array-except-self/) | [ProductExceptSelf.java](src/array_string/ProductExceptSelf.java) | Medium |
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| Array / String | [Increasing Triplet Subsequence](https://leetcode.com/problems/increasing-triplet-subsequence/) | [IncreasingTriplet.java](src/array_string/IncreasingTriplet.java) | Medium |
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| Array / String | [String Compression](https://leetcode.com/problems/string-compression/) | [StringCompressor.java](src/array_string/StringCompressor.java) | Medium |
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| Two Pointers | [Move Zeroes](https://leetcode.com/problems/move-zeroes/) | [ZeroMover.java](src/two_pointers/ZeroMover.java) | Easy |
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| Two Pointers | [Is Subsequence](https://leetcode.com/problems/is-subsequence/) | [SubsequenceChecker.java](src/two_pointers/SubsequenceChecker.java) | Easy |
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| Two Pointers | [Container With Most Water](https://leetcode.com/problems/container-with-most-water/) | [WaterContainer.java](src/two_pointers/WaterContainer.java) | Medium |
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| Two Pointers | [Max Number of K-Sum Pairs](https://leetcode.com/problems/max-number-of-k-sum-pairs/) | [MaxOperationsCounter.java](src/two_pointers/MaxOperationsCounter.java) | Medium |
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| Sliding Window | [Maximum Average Subarray I](https://leetcode.com/problems/maximum-average-subarray-i/) | [MaxAverageSubarrayCalculator.java](src/sliding_window/MaxAverageSubarrayCalculator.java) | Easy |
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| Sliding Window | [Maximum Number of Vowels in a Substring of Given Length](https://leetcode.com/problems/maximum-number-of-vowels-in-a-substring-of-given-length/) | [MaxVowelSubstringCalculator.java](src/sliding_window/MaxVowelSubstringCalculator.java) | Medium |
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| Sliding Window | [Max Consecutive Ones III](https://leetcode.com/problems/max-consecutive-ones-iii/) | [BinaryArrayMaxOnesCalculator.java](src/sliding_window/BinaryArrayMaxOnesCalculator.java) | Medium |
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| Sliding Window | [Longest Subarray of 1's After Deleting One Element](https://leetcode.com/problems/longest-subarray-of-1s-after-deleting-one-element/) | [BinarySubarrayCalculator.java](src/sliding_window/BinarySubarrayCalculator.java) | Medium |
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| Prefix Sum | [Find the Highest Altitude](https://leetcode.com/problems/find-the-highest-altitude/) | [AltitudeCalculator.java](src/prefix_sum/AltitudeCalculator.java) | Easy |
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| Prefix Sum | [Find Pivot Index](https://leetcode.com/problems/find-pivot-index/) | [PivotFinder.java](src/prefix_sum/PivotFinder.java) | Easy |
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| Hash Map / Set | [Find the Difference of Two Arrays](https://leetcode.com/problems/find-the-difference-of-two-arrays/) | [ArraysDifferenceFinder.java](src/hash_map_set/ArraysDifferenceFinder.java) | Easy |
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| Hash Map / Set | [Unique Number of Occurrences](https://leetcode.com/problems/unique-number-of-occurrences/) | [UniqueOccurrenceChecker.java](src/hash_map_set/UniqueOccurrenceChecker.java) | Easy |
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| Hash Map / Set | [Determine if Two Strings Are Close](https://leetcode.com/problems/determine-if-two-strings-are-close/) | [StringCloseChecker.java](src/hash_map_set/StringCloseChecker.java) | Medium |
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| Hash Map / Set | [Equal Row and Column Pairs](https://leetcode.com/problems/equal-row-and-column-pairs/) | [MatrixPairs.java](src/hash_map_set/MatrixPairs.java) | Medium |
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| ... | ... | ... | ... |
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| Trie | [Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree) | [PrefixTrie.java](src/trie/PrefixTrie.java) | Medium |
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| Trie | [Search Suggestions System](https://leetcode.com/problems/search-suggestions-system) | [SearchSuggestionsSystem.java](src/trie/SearchSuggestionsSystem.java) | Medium |
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| ... | ... | ... | ... |
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## Usage
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‎src/hash_map_set/ArraysDifferenceFinder.java

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package hash_map_set;
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import java.util.*;
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import java.util.stream.Collectors;
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public class ArraysDifferenceFinder {
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public List<List<Integer>> findDifference(int[] nums1, int[] nums2) {
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Set<Integer> set1 = new HashSet<>(Arrays.stream(nums1).boxed().collect(Collectors.toSet())); // convert nums1 to set
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Set<Integer> set2 = new HashSet<>(Arrays.stream(nums2).boxed().collect(Collectors.toSet())); // convert nums2 to set
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List<Integer> diff1 = set1.stream().filter(num -> !set2.contains(num)).collect(Collectors.toList()); // nums1 diff nums2
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List<Integer> diff2 = set2.stream().filter(num -> !set1.contains(num)).collect(Collectors.toList()); // nums2 diff nums1
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return Arrays.asList(diff1, diff2);
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}
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public static void main(String[] args) {
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ArraysDifferenceFinder finder = new ArraysDifferenceFinder();
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List<List<Integer>> result1 = finder.findDifference(new int[] {1,2,3}, new int[] {2,4,6});
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assert result1.get(0).equals(Arrays.asList(1,3)) && result1.get(1).equals(Arrays.asList(4,6)) : "Test case 1 failed";
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List<List<Integer>> result2 = finder.findDifference(new int[] {1,2,3,3}, new int[] {1,1,2,2});
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assert result2.get(0).equals(Arrays.asList(3)) && result2.get(1).isEmpty() : "Test case 2 failed";
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System.out.println("All test cases passed!");
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}
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}

‎src/hash_map_set/MatrixPairs.java

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package hash_map_set;
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import java.util.HashMap;
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import java.util.Map;
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import java.util.Arrays;
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public class MatrixPairs {
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public int countEqualPairs(int[][] matrix) {
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Map<String, Integer> rowRepresentationCount = new HashMap<>();
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for (int[] row : matrix) {
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String rowString = Arrays.toString(row);
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rowRepresentationCount.put(rowString, rowRepresentationCount.getOrDefault(rowString, 0) + 1);
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}
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int equalPairCount = 0;
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int matrixSize = matrix.length;
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for (int colIndex = 0; colIndex < matrixSize; colIndex++) {
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int[] column = new int[matrixSize];
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for (int rowIndex = 0; rowIndex < matrixSize; rowIndex++) {
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column[rowIndex] = matrix[rowIndex][colIndex];
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}
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String colString = Arrays.toString(column);
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equalPairCount += rowRepresentationCount.getOrDefault(colString, 0);
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}
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return equalPairCount;
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}
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public static void main(String[] args) {
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MatrixPairs checker = new MatrixPairs();
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assert checker.countEqualPairs(new int[][]{{3,2,1},{1,7,6},{2,7,7}}) == 1 : "Test case 1 failed";
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assert checker.countEqualPairs(new int[][]{{3,1,2,2},{1,4,4,5},{2,4,2,2},{2,4,2,2}}) == 3 : "Test case 2 failed";
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System.out.println("All test cases passed!");
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}
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}

‎src/hash_map_set/StringCloseChecker.java

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package hash_map_set;
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import java.util.Arrays;
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public class StringCloseChecker {
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private static final int LETTER_COUNT = 26;
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public boolean areStringsClose(String str1, String str2) {
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int[] freq1 = new int[LETTER_COUNT];
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int[] freq2 = new int[LETTER_COUNT];
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for (char ch : str1.toCharArray()) {
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freq1[ch - 'a']++;
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}
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for (char ch : str2.toCharArray()) {
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freq2[ch - 'a']++;
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}
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for (int i = 0; i < LETTER_COUNT; i++) {
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if (freq1[i] == 0 && freq2[i] != 0 || freq1[i] != 0 && freq2[i] == 0) {
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return false;
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}
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}
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Arrays.sort(freq1);
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Arrays.sort(freq2);
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return Arrays.equals(freq1, freq2);
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}
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public static void main(String[] args) {
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StringCloseChecker checker = new StringCloseChecker();
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assert checker.areStringsClose("abc", "bca") : "Test case 1 failed";
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assert !checker.areStringsClose("a", "aa") : "Test case 2 failed";
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assert checker.areStringsClose("cabbba", "abbccc") : "Test case 3 failed";
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System.out.println("All test cases passed!");
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}
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}

‎src/hash_map_set/UniqueOccurrenceChecker.java

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package hash_map_set;
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import java.util.*;
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public class UniqueOccurrenceChecker {
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public boolean hasUniqueOccurrences(int[] numbers) {
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Map<Integer, Integer> numberCounts = new HashMap<>();
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for (int num : numbers) {
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numberCounts.put(num, numberCounts.getOrDefault(num, 0) + 1);
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}
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Set<Integer> uniqueCounts = new HashSet<>(numberCounts.values());
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return numberCounts.size() == uniqueCounts.size();
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}
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public static void main(String[] args) {
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UniqueOccurrenceChecker checker = new UniqueOccurrenceChecker();
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assert checker.hasUniqueOccurrences(new int[]{1,2,2,1,1,3}) : "Test case 1 failed";
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assert !checker.hasUniqueOccurrences(new int[]{1,2}) : "Test case 2 failed";
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assert checker.hasUniqueOccurrences(new int[]{-3,0,1,-3,1,1,1,-3,10,0}) : "Test case 3 failed";
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System.out.println("All test cases passed!");
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}
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}

‎src/prefix_sum/AltitudeCalculator.java

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package prefix_sum;
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public class AltitudeCalculator {
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public int largestAltitude(int[] gain) {
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int currentAltitude = 0;
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int maxAltitude = 0;
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for (int altitudeChange : gain) {
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currentAltitude += altitudeChange;
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maxAltitude = Math.max(maxAltitude, currentAltitude);
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}
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return maxAltitude;
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}
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public static void main(String[] args) {
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AltitudeCalculator calculator = new AltitudeCalculator();
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assert calculator.largestAltitude(new int[]{-5, 1, 5, 0, -7}) == 1 : "Test case 1 failed";
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assert calculator.largestAltitude(new int[]{-4, -3, -2, -1, 4, 3, 2}) == 0 : "Test case 2 failed";
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System.out.println("All test cases passed!");
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}
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}

‎src/prefix_sum/PivotFinder.java

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package prefix_sum;
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import java.util.Arrays;
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public class PivotFinder {
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public int pivotIndex(int[] nums) {
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int total = Arrays.stream(nums).sum();
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int leftSum = 0;
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for (int i = 0; i < nums.length; i++) {
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if (leftSum == total - leftSum - nums[i]) {
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return i;
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}
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leftSum += nums[i];
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}
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return -1;
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}
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public static void main(String[] args) {
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PivotFinder finder = new PivotFinder();
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assert finder.pivotIndex(new int[]{1,7,3,6,5,6}) == 3 : "Test case 1 failed";
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assert finder.pivotIndex(new int[]{1,2,3}) == -1 : "Test case 2 failed";
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assert finder.pivotIndex(new int[]{2,1,-1}) == 0 : "Test case 3 failed";
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System.out.println("All test cases passed!");
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}
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}

‎src/sliding_window/BinaryArrayMaxOnesCalculator.java

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package sliding_window;
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public class BinaryArrayMaxOnesCalculator {
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public int longestOnes(int[] nums, int flipsAllowed) {
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int left = 0, right;
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for (right = 0; right < nums.length; ++right) {
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if (nums[right] == 0) flipsAllowed--;
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if (flipsAllowed < 0 && nums[left++] == 0) flipsAllowed++;
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}
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return right - left;
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}
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public static void main(String[] args) {
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BinaryArrayMaxOnesCalculator calculator = new BinaryArrayMaxOnesCalculator();
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assert calculator.longestOnes(new int[]{1,1,1,0,0,0,1,1,1,1,0}, 2) == 6 : "Test case 1 failed";
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assert calculator.longestOnes(new int[]{0,0,1,1,0,0,1,1,1,0,1,1,0,0,0,1,1,1,1}, 3) == 10 : "Test case 2 failed";
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System.out.println("All test cases passed!");
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}
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}

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