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]
]

confused what

"{1,#,2,3}"

means? >
read more on how binary tree is serialized on OJ.

/**
 * 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>> res;
        if(root == NULL)
            return res;
        stack<TreeNode*> use;
        queue<TreeNode*> save;
        save.push(root);
        bool flag = false;
        while(!save.empty())
        {
            vector<int> t;
            while(!save.empty())
            {
                use.push(save.front());
                save.pop();
            }
            while(!use.empty())
            {
                TreeNode* temp = use.top();
                use.pop();
                t.push_back(temp->val);
                if(flag == true)
                {
                    if(temp->right != NULL)
                        save.push(temp->right);
                    if(temp->left != NULL)
                        save.push(temp->left);
                }
                else
                {
                    if(temp->left != NULL)
                        save.push(temp->left);
                    if(temp->right != NULL)
                        save.push(temp->right);
                }
            }
            flag = (!flag);
            res.push_back(t);
        }
        return res;
    }
};