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]
]
 1 public class Solution {
 2         public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
 3             ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
 4             if(root==null) return res;
 5             LinkedList<TreeNode> cur = new LinkedList<TreeNode>();
 6             cur.offer(root);
 7             int level = 0;
 8             while(!cur.isEmpty()){
 9                 LinkedList<TreeNode> next = new LinkedList<TreeNode>();
10                 ArrayList<Integer> output = new ArrayList<Integer>();
11                 for(TreeNode n:cur){
12                     output.add(n.val);
13                     if(n.left!=null){
14                         next.offer(n.left);
15                     }
16                     if(n.right!=null){
17                         next.offer(n.right);
18                     }
19                 }
20                 if(level%2==1){
21                      Collections.reverse(output);
22                 }
23                 res.add(output);
24                 cur = next;
25                 level++;
26             }
27             return res;
28         }
29     }
View Code

 

 
posted @ 2014-02-06 15:31  krunning  阅读(125)  评论(0)    收藏  举报