面试题23. 从上往下打印二叉树

题目描述
从上往下打印出二叉树的每个节点，同层节点从左至右打印。

思路：bfs

本题的考点是树的广搜

Java 代码：

import java.util.ArrayList;
import java.util.LinkedList;

public class Solution
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {

        ArrayList<Integer> arr = new ArrayList();
        if(root == null) {
            return arr;
        }

        LinkedList<TreeNode> nodes = new LinkedList();
        nodes.add(root);
        TreeNode node;

        while(!nodes.isEmpty()) {
            int size = nodes.size();            
            for(int i = 0; i < size; i++) {
                node = nodes.get(0);
                arr.add(node.val);
                if(node.left != null) {
                    nodes.add(node.left);
                }
                if(node.right != null) {
                    nodes.add(node.right);
                }                                
                nodes.remove(0);
            }
        }        
        return

Python 代码：

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # 返回从上到下每个节点值列表，例：[1,2,3]
    def PrintFromTopToBottom(self, root):     
        arr = []
        result = []
        if not root:
            return result
        arr.append(root)
        while arr:
            size = len(arr)
            for i in range(0, size):
                node = arr[0]
                result.append(node.val)
                if node.left:
                    arr.append(node.left)
                if node.right:
                    arr.append(node.right)
                del(arr[0])
        return