Java [Leetcode 102]Binary Tree Level Order Traversal

题目描述：

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / 
  9  20
    /  
   15   7

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

解题思路：

运用广度优先搜索方法，同http://www.cnblogs.com/zihaowang/p/5149745.html类似。

代码如下：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new LinkedList<List<Integer>>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        if(root == null)
        	return result;
        queue.offer(root);
        while(!queue.isEmpty()){
        	int m = queue.size();
        	List<Integer> list = new LinkedList<Integer>();
        	for(int i = 0; i < m; i++){
        		if(queue.peek().left != null)
        			queue.offer(queue.peek().left);
        		if(queue.peek().right != null)
        			queue.offer(queue.peek().right);
        		list.add(queue.poll().val);
        	}
        	result.add(new LinkedList<Integer>(list));
        }
        return result;
    }
}