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]
]

Solution:

 1 /**
 2  * Definition for binary tree
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
12         // Start typing your Java solution below
13         // DO NOT write main() function
14         ArrayList<ArrayList<Integer>> ans = new ArrayList<ArrayList<Integer>>();
15         LinkedList<TreeNode> currentLevel = new LinkedList<TreeNode>();
16         int level = 0;
17         if(root != null) 
18             currentLevel.add(root);
19         while(!currentLevel.isEmpty()){
20             LinkedList<TreeNode> nextLevel = new LinkedList<TreeNode>();
21             ArrayList<Integer> list = new ArrayList<Integer>();
22             for(TreeNode node : currentLevel){
23                 list.add(node.val);
24                 if(node.left != null)
25                     nextLevel.add(node.left);
26                 if(node.right != null)
27                     nextLevel.add(node.right);
28             }
29             if(level%2 == 1)
30                 Collections.reverse(list);
31             ans.add(list);
32             currentLevel = nextLevel;
33             level++;
34         }
35         return ans;
36     }
37 }

 

posted @ 2013-01-26 03:07  cradle  阅读(799)  评论(0)  编辑  收藏  举报