【leetcode】Binary Tree Zigzag Level Order Traversal

Question ：

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its zigzag level order traversal as:

[  [3],  [20,9],  [15,7]]

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

Anwser 1 ：       

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<vector<int>> ret;        if(root == NULL) return ret;                vector<int> vec;        stack<TreeNode *> Q;        stack<TreeNode *> Q2;   // extra space        bool flag = true;                Q.push(root);        while(!Q.empty()){            TreeNode *tmp = Q.top();            Q.pop();                        if(tmp != NULL){                vec.push_back(tmp->val);                if(flag){                    if(tmp->left) Q2.push(tmp->left);                    if(tmp->right) Q2.push(tmp->right);                } else {                    if(tmp->right) Q2.push(tmp->right);                    if(tmp->left) Q2.push(tmp->left);                }            }                        if(Q.empty()){      // one row end                ret.push_back(vec);                vec.clear();                flag = !flag;                swap(Q, Q2);            }        }        return ret;    }};