[LeetCode] Binary Tree Zigzag Level Order Traversal

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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function

vector<vector<int> > ans;
if ( !root )
return ans;

int flag=0;

deque<int> level;
vector<TreeNode*> cur;
vector<TreeNode*> next;
cur.push_back(root);
while(!cur.empty())
{
level.clear();
next.clear();

int start=0;
int end=cur.size();
while(start!=end)
{
TreeNode* tmp=cur[start];
if ( flag )
level.push_front(tmp->val);
else
level.push_back(tmp->val);
if ( tmp->left )
next.push_back(tmp->left);
if ( tmp->right)
next.push_back(tmp->right);
start++;
}
ans.push_back(vector<int>(level.begin(),level.end()));
cur=next;
flag=1-flag;
}
return ans;
}
};