Binary Tree Zigzag Level Order Traversal

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]]

/** * 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) {        vector<vector<int> > result;        if (root == NULL)        {            return result;        }        queue<TreeNode*> buf;        bool left = true;        buf.push(root);        buf.push(NULL);        vector<int> temp;        stack<int> tempRight;        while (!buf.empty())        {            TreeNode *front = buf.front();            if (front != NULL)            {                if (left)                {                    temp.push_back(front->val);                }                else                {                    tempRight.push(front->val);                }                if (front->left)                {                    buf.push(front->left);                }                if (front->right)                {                    buf.push(front->right);                }            }            else            {                if (!left)                {                    while (!tempRight.empty())                    {                        int top = tempRight.top();                        temp.push_back(top);                        tempRight.pop();                    }                }                result.push_back(temp);                temp.clear();                if (buf.size() > 1)                {                    if (left)                    {                        left = false;                    }                    else                    {                        left = true;                    }                    buf.push(NULL);                }            }            buf.pop();        }        return result;    }};