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},

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.

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
13         vector<vector<int>> result;
14         traversal(root,1,result,true);
15         return result;
16     }
17     
18     void traversal(TreeNode *node,int level,vector<vector<int>> &result,bool left_toright)
19     {
20         
21         if(node==NULL) return;
22         if(level>result.size())
23             result.push_back(vector<int>());  //不可以缺少
24         if(left_toright)
25         {
26             result[level-1].push_back(node->val);
27         }
28         else{
29             result[level-1].insert(result[level-1].begin(),node->val);
30         }
31         
32         traversal(node->left,level+1,result,!left_toright);
33         traversal(node->right,level+1,result,!left_toright);
34         
35         
36     }
37     
38 };