LeetCode 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> > vecArray;
if (NULL == root)
return vecArray;

bool flag = false;
queue<TreeNode *> queueNode;
queueNode.push(root);
while (!queueNode.empty()) {
vector<int> ivec;
int n = queueNode.size();
TreeNode *pTree;
for (int i = 0; i < n; i++) {
pTree = queueNode.front();
queueNode.pop();
if (pTree->left)
queueNode.push(pTree->left);
if (pTree->right)
queueNode.push(pTree->right);
ivec.push_back(pTree->val);
}
if (flag)
exch(ivec);
flag = !flag;
vecArray.push_back(ivec);
}
return vecArray;
}

void exch(vector<int> &ivec) {
int i = 0, j = ivec.size() - 1;
while (i < j) {
int temp = ivec[i];
ivec[i++] = ivec[j];
ivec[j--] = temp;
}
}
};