【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;
    }
};