Binary Tree Zigzag Level 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]
] 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

Analyse: The same as Binary Tree Level Order Traversal except that we need a boolean variable to determine whether the current level need reverse.

 

1. Recursion

    Runtime: 4ms

 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         zigzag(root, 0, true, result);
15         return result;
16     }
17     void zigzag(TreeNode* root, int level, bool l2r, vector<vector<int> >& result){
18         if(!root) return;
19         if(level == result.size()) result.push_back(vector<int> ());
20         
21         if(l2r) result[level].push_back(root->val);
22         else result[level].insert(result[level].begin(), root->val);
23         
24         zigzag(root->left, level + 1, !l2r, result);
25         zigzag(root->right, level + 1, !l2r, result);
26     }
27 };

2. Iteration

　 Runtime: 4ms.

 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         if(!root) return result;
15         queue<TreeNode* > qu;
16         qu.push(root);
17         bool l2r = false;
18         
19         while(!qu.empty()){
20             int n = qu.size();
21             l2r = !l2r;
22             
23             vector<int> level;
24             while(n--){
25                 TreeNode* temp = qu.front();
26                 qu.pop();
27                 
28                 if(l2r) level.push_back(temp->val);
29                 else level.insert(level.begin(), temp->val); //if the boolean variable is not true, do reverse-sequence 
30                 
31                 if(temp->left) qu.push(temp->left);
32                 if(temp->right) qu.push(temp->right);
33             }
34             result.push_back(level);
35         }
36         return result;
37     }
38 };

 

 

posted @ 2015-07-28 10:41  amazingzoe  阅读(179)  评论(0)  编辑  收藏  举报