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]
]
在level的基础上加一个bool变量来记录是否轮到left_toright.

/**
* Definition for a binary tree node.
* 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>> result;
traversal(root,1,result,true);
return result;
}

void traversal(TreeNode *node,int level,vector<vector<int>> &result,bool left_toright)
{

if(node==NULL) return;
if(level>result.size())
result.push_back(vector<int>());  //不可以缺少
if(left_toright)
{
result[level-1].push_back(node->val);
}
else{
result[level-1].insert(result[level-1].begin(),node->val);
}

traversal(node->left,level+1,result,!left_toright);
traversal(node->right,level+1,result,!left_toright);

}

};