題目：

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]
]

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below. 

Here's an example: 

   1
  / \
 2   3
    /
   4
    \
     5

思路：

跟前面42題一樣，只是最後加的時候不加在頭部即可。

程式碼：

import 
 
java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.Queue;
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public 
 
ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(root==null){
            return result;
        }
        Queue<TreeNode> que = new LinkedList<TreeNode>();
        que.add(root);
        while(!que.isEmpty()){
            int count = que.size();
            ArrayList<Integer> list = new ArrayList<Integer>();
            for(int i=0;i<count;i++){
                TreeNode node = que.poll();
                list.add(node.val);
                if(node.left!=null){
                    que.add(node.left);
                }
                if(node.right != null){
                    que.add(node.right);
                }
            }
            result.add(list);
        }
        return result;
    }
}

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]
]

The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below. 

Here's an example: 

   1
  / \
 2   3
    /
   4
    \
     5