solve problem Combinations

Author: P-ppc

README.md

@@ -225,6 +225,7 @@ All solutions will be accepted!
 |40|[Combination Sum II](https://leetcode-cn.com/problems/combination-sum-ii/description/)|[java/py/js](./algorithms/CombinationSumII)|Medium|
 |216|[Combination Sum III](https://leetcode-cn.com/problems/combination-sum-iii/description/)|[java/py/js](./algorithms/CombinationSumIII)|Medium|
 |46|[Permutations](https://leetcode-cn.com/problems/permutations/description/)|[java/py/js](./algorithms/Permutations)|Medium|
+|77|[Combinations](https://leetcode-cn.com/problems/combinations/description/)|[java/py/js](./algorithms/Combinations)|Medium|
 
 # Database
 |#|Title|Solution|Difficulty|

algorithms/Combinations/README.md

@@ -0,0 +1,2 @@
+# Combinations
+This problem is easy to solve by Backtracking Algorithm

algorithms/Combinations/Solution.java

@@ -0,0 +1,19 @@
+class Solution {
+    public List<List<Integer>> combine(int n, int k) {
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        backTrack(1, n, k, new ArrayList<Integer>(), res);
+        return res;
+    }
+    
+    public void backTrack(int beg, int n, int k, List<Integer> temp, List<List<Integer>> res) {
+        if (temp.size() == k) {
+            res.add(temp);
+        } else {
+            for (int i = beg; i <= n; i++) {
+                List<Integer> t = new ArrayList<Integer>(temp);
+                t.add(i);
+                backTrack(i + 1, n, k, t, res);
+            }
+        }
+    }
+}

algorithms/Combinations/solution.js

@@ -0,0 +1,23 @@
+/**
+ * @param {number} n
+ * @param {number} k
+ * @return {number[][]}
+ */
+var combine = function(n, k) {
+    let res = []
+    backTrack(0, Array.from({ length: n }, (v, k) => k + 1), k, [], res)
+    return res
+};
+
+var backTrack = function(beg, nums, k, temp, res) {
+    if (temp.length === k) {
+        res.push(temp)
+    } else {
+        for (let i = beg; i < nums.length; i++) {
+            let num = nums[i],
+                t = temp.slice()
+            t.push(num)
+            backTrack(i + 1, nums, k, t, res)
+        }
+    }
+}

algorithms/Combinations/solution.py

@@ -0,0 +1,19 @@
+class Solution(object):
+    def combine(self, n, k):
+        """
+        :type n: int
+        :type k: int
+        :rtype: List[List[int]]
+        """
+        res = []
+        self.backtrack(0, range(1, n + 1), k, [], res)
+        return res
+    
+    def backtrack(self, beg, nums, k, temp, res):
+        if len(temp) == k:
+            res.append(temp)
+        else:
+            for i in xrange(beg, len(nums)):
+                t = temp[:]
+                t.append(nums[i])
+                self.backtrack(i + 1, nums, k, t, res)