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;
}
};