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);
        
        
    }
    
};