【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=1;
       
        bool isOdd=true;
       
        while(!q.empty())
        {
            vector<int> cur;
            int nextLevelCount=0;
            for(int i=0;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;
       
    }
};