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Author: BaffinLee

001-100/77. Combinations.md

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+# 77. Combinations
+
+- Difficulty: Medium.
+- Related Topics: Backtracking.
+- Similar Questions: Combination Sum, Permutations.
+
+## Problem
+
+Given two integers **n** and **k**, return all possible combinations of **k** numbers out of 1 ... **n**.
+
+**Example:**
+
+```
+Input: n = 4, k = 2
+Output:
+[
+  [2,4],
+  [3,4],
+  [2,3],
+  [1,2],
+  [1,3],
+  [1,4],
+]
+```
+
+## Solution
+
+```javascript
+/**
+ * @param {number} n
+ * @param {number} k
+ * @return {number[][]}
+ */
+var combine = function(n, k) {
+  if (n < k || k < 1) return [];
+  
+  var res = [];
+  
+  helper(res, [], 0, n, k);
+  
+  return res;
+};
+
+var helper = function (res, now, start, n, k) {
+  if (k === 0) {
+    res.push(Array.from(now));
+    return;
+  }
+
+  for (var i = start; i < n; i++) {
+    now.push(i + 1)
+    helper(res, now, i + 1, n, k - 1);
+    now.pop();
+  }
+};
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity :
+* Space complexity :

001-100/78. Subsets.md

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+# 78. Subsets
+
+- Difficulty: Medium.
+- Related Topics: Array, Backtracking, Bit Manipulation.
+- Similar Questions: Subsets II, Generalized Abbreviation, Letter Case Permutation.
+
+## Problem
+
+Given a set of **distinct** integers, **nums**, return all possible subsets (the power set).
+
+**Note:** The solution set must not contain duplicate subsets.
+
+**Example:**
+
+```
+Input: nums = [1,2,3]
+Output:
+[
+  [3],
+  [1],
+  [2],
+  [1,2,3],
+  [1,3],
+  [2,3],
+  [1,2],
+  []
+]
+```
+
+## Solution
+
+```javascript
+
+```
+
+**Explain:**
+
+nope.
+
+**Complexity:**
+
+* Time complexity : O(n).
+* Space complexity : O(n).