feat: solve No.34

Author: BaffinLee

001-100/34. Find First and Last Position of Element in Sorted Array.md

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+# 34. Find First and Last Position of Element in Sorted Array
+
+- Difficulty: Medium.
+- Related Topics: Array, Binary Search.
+- Similar Questions: First Bad Version, Plates Between Candles, Find Target Indices After Sorting Array.
+
+## Problem
+
+Given an array of integers `nums` sorted in non-decreasing order, find the starting and ending position of a given `target` value.
+
+If `target` is not found in the array, return `[-1, -1]`.
+
+You must write an algorithm with `O(log n)` runtime complexity.
+
+ 
+Example 1:
+```
+Input: nums = [5,7,7,8,8,10], target = 8
+Output: [3,4]
+```Example 2:
+```
+Input: nums = [5,7,7,8,8,10], target = 6
+Output: [-1,-1]
+```Example 3:
+```
+Input: nums = [], target = 0
+Output: [-1,-1]
+```
+ 
+**Constraints:**
+
+
+	
+- `0 <= nums.length <= 105`
+	
+- `-109 <= nums[i] <= 109`
+	
+- `nums` is a non-decreasing array.
+	
+- `-109 <= target <= 109`
+
+
+
+## Solution
+
+```javascript
+/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+var searchRange = function(nums, target) {
+    var index = findIndex(nums, target);
+    return index === -1
+        ? [-1, -1]
+        : [findLeft(nums, target, index), findRight(nums, target, index)];
+};
+
+var findIndex = function(nums, target) {
+    var left = 0;
+    var right = nums.length - 1;
+    while (left <= right) {
+        var mid = left + Math.floor((right - left) / 2);
+        if (nums[mid] === target) {
+            return mid;
+        } else if (nums[mid] > target) {
+            right = mid - 1;
+        } else {
+            left = mid + 1;
+        }
+    }
+    return -1;
+};
+
+var findLeft = function(nums, target, index) {
+    var left = 0;
+    var right = index;
+    while (left < right) {
+        var mid = left + Math.floor((right - left) / 2);
+        if (nums[mid] === target) {
+            right = mid;
+        } else {
+            left = mid + 1;
+        }
+    }
+    return left;
+};
+
+var findRight = function(nums, target, index) {
+    var left = index;
+    var right = nums.length - 1;
+    while (left < right) {
+        var mid = left + Math.ceil((right - left) / 2);
+        if (nums[mid] === target) {
+            left = mid;
+        } else {
+            right = mid - 1;
+        }
+    }
+    return right;
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(log(n)).
+* Space complexity : O(1).