Create 103. Binary Tree Zigzag Level Order Traversal.md

solutions/103. Binary Tree Zigzag Level Order Traversal.md

@@ -0,0 +1,34 @@
+# [103. Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)
+# 思路
+这题是[102层序遍历](https://leetcode.com/problems/binary-tree-level-order-traversal/)的推广，我们只需要在合适的层翻转原始访问顺序就可以了，
+普通的层序遍历可参考[102题解](https://github.com/ShusenTang/LeetCode/blob/master/solutions/102.%20Binary%20Tree%20Level%20Order%20Traversal.md)，这里不再赘述。
+
+# C++
+``` C++
+class Solution {
+public:
+    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
+        vector<vector<int>>res;
+        if(root == NULL) return res;
+        queue<TreeNode *>q;
+        TreeNode *p;
+        
+        bool left_to_right = true;
+        q.push(root);
+        while(!q.empty()){
+            vector<int>level;
+            for(int i = q.size(); i > 0; i--){
+                p = q.front();
+                q.pop();
+                level.push_back(p -> val);
+                if(p -> left) q.push(p -> left);
+                if(p -> right) q.push(p -> right);
+            }
+            if(!left_to_right) reverse(level.begin(), level.end()); // 翻转原始访问顺序
+            left_to_right = !left_to_right;               
+            res.push_back(level);
+        }
+        return res;
+    }
+};
+```