【leetcode】Binary Tree Zigzag Level Order Traversal

Question ：

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.

Anwser 1 ：

/**
* 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 == NULL) return ret;

vector<int> vec;
stack<TreeNode *> Q;
stack<TreeNode *> Q2;   // extra space
bool flag = true;

Q.push(root);
while(!Q.empty()){
TreeNode *tmp = Q.top();
Q.pop();

if(tmp != NULL){
vec.push_back(tmp->val);
if(flag){
if(tmp->left) Q2.push(tmp->left);
if(tmp->right) Q2.push(tmp->right);
} else {
if(tmp->right) Q2.push(tmp->right);
if(tmp->left) Q2.push(tmp->left);
}
}

if(Q.empty()){      // one row end
ret.push_back(vec);
vec.clear();
flag = !flag;
swap(Q, Q2);
}
}
return ret;
}
};