【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]
]

 

 

利用队列，进行层次遍历

根据层的奇偶性，判断是否需要翻转

 

 

 /**
  * 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) {

         vector<vector<int> > result;
         queue<TreeNode*> q;

         if(root==NULL) return result;
         q.push(root);

         int numNode=;

         bool isOdd=true;

         while(!q.empty())
         {
             vector<int> cur;
             int nextLevelCount=;
             for(int i=;i<numNode;i++)
             {
                 TreeNode *node=q.front();
                 cur.push_back(node->val);
                 q.pop();
                 if(node->left!=NULL)
                 {
                     q.push(node->left);
                     nextLevelCount++;
                 }

                 if(node->right!=NULL)
                 {
                     q.push(node->right);
                     nextLevelCount++;
                 }
             }

             if(isOdd)
             {
                 result.push_back(cur);
             }
             else
             {
                 reverse(cur.begin(),cur.end());
                 result.push_back(cur);
             }

             isOdd=!isOdd;

             numNode=nextLevelCount;
         }

         return result;

     }
 };