从上往下打印二叉树

image.png

广度优先遍历，建一个队列，每次压入左右子节点，知道队列为空。
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):
        # write code here
        self.st = [root]
        self.ans = []
        if root == None:
            return self.ans
        while self.st != None:
            if len(self.st) == 0:
                break
            p = self.st[0]
            del self.st[0]
            if p.left:
                self.st.append(p.left)
            if p.right:
                self.st.append(p.right)
            self.ans.append(p.val)
        return self.ans