【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;

    }

};