Binary Tree Zigzag Level Order Traversal

题目

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}"

 

思路

类似于层次遍历

 

/**
* 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> >  result;
if(!root)
return result;

queue<TreeNode *> curqueue, nextqueue;
TreeNode * ptr;
vector<int> tmpvector;
bool flag = false;

nextqueue.push(root);
while(true)
{
tmpvector.clear();
curqueue = nextqueue;
while(!nextqueue.empty()) nextqueue.pop();
while(!curqueue.empty())
{
ptr = curqueue.front();
curqueue.pop();
tmpvector.push_back(ptr->val);
if(ptr->left!=NULL)
nextqueue.push(ptr->left);
if(ptr->right!=NULL)
nextqueue.push(ptr->right);
}
if(tmpvector.size()>0)
{
if(flag) {
reverse(tmpvector.begin(),tmpvector.end());
}
flag = !flag;
result.push_back(tmpvector);
}
else
break;
}
}
};

其他的层次遍历方法，请参考： 
Binary Tree Level Order Traversal