添加0077.组合python2版本

problems/0077.组合.md

@@ -368,6 +368,34 @@ class Solution {
 }
 ```
 
+Python2:
+```python
+class Solution(object):
+    def combine(self, n, k):
+        """
+        :type n: int
+        :type k: int
+        :rtype: List[List[int]]
+        """
+        result = []
+        path = []
+        def backtracking(n, k, startidx):
+            if len(path) == k:
+                result.append(path[:])
+                return
+
+            # 剪枝， 最后k - len(path)个节点直接构造结果，无需递归
+            last_startidx = n - (k - len(path)) + 1
+            result.append(path + [idx for idx in range(last_startidx, n + 1)])
+
+            for x in range(startidx, last_startidx):
+                path.append(x)
+                backtracking(n, k, x + 1)  # 递归
+                path.pop()  # 回溯
+
+        backtracking(n, k, 1)
+        return result
+```
 
 Python：
 ```python3