[LeetCode] Binary Tree Zigzag Level Order Traversal

/**
* 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) return ret;

queue<TreeNode *> q;
q.push(root);
q.push(NULL);

vector<int> v;

while (!q.empty()) {
TreeNode *node = q.front();
q.pop();

if (node) {
v.push_back(node->val);
if (node->left)  q.push(node->left);
if (node->right) q.push(node->right);
} else {
if (ret.size() % 2 == 1)
reverse(v.begin(), v.end());

ret.push_back(v);
v.clear();

if (!q.empty()) q.push(NULL);
}
}

return ret;
}
};