[leetcode] 655. Print Binary Tree @ python

最新推荐文章于 2022-12-20 17:05:20 发布

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最新推荐文章于 2022-12-20 17:05:20 发布

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本文链接：https://blog.youkuaiyun.com/danspace1/article/details/86605281

本文介绍了一种将二叉树以二维字符串数组形式打印的方法，遵循特定规则确保树的结构完整呈现。算法通过递归方式，根据二叉树高度确定数组大小，再将根节点及其子树正确放置于数组中，实现美观且清晰的二叉树可视化。

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原题

Print a binary tree in an m*n 2D string array following these rules:

The row number m should be equal to the height of the given binary tree.
The column number n should always be an odd number.
The root node’s value (in string format) should be put in the exactly middle of the first row it can be put. The column and the row where the root node belongs will separate the rest space into two parts (left-bottom part and right-bottom part). You should print the left subtree in the left-bottom part and print the right subtree in the right-bottom part. The left-bottom part and the right-bottom part should have the same size. Even if one subtree is none while the other is not, you don’t need to print anything for the none subtree but still need to leave the space as large as that for the other subtree. However, if two subtrees are none, then you don’t need to leave space for both of them.
Each unused space should contain an empty string “”.
Print the subtrees following the same rules.
Example 1:
Input:
1
/
2
Output:
[["", “1”, “”],
[“2”, “”, “”]]
Example 2:
Input:
1
/
2 3

4
Output:
[["", “”, “”, “1”, “”, “”, “”],
["", “2”, “”, “”, “”, “3”, “”],
["", “”, “4”, “”, “”, “”, “”]]
Example 3:
Input:
1
/
2 5
/
3
/
4
Output:

[["", “”, “”, “”, “”, “”, “”, “1”, “”, “”, “”, “”, “”, “”, “”]
["", “”, “”, “2”, “”, “”, “”, “”, “”, “”, “”, “5”, “”, “”, “”]
["", “3”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”]
[“4”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”, “”]]

解法

观察数组可得, 数组的长度是二叉树的高度, 子数组的长度是2**h-1. 因此定义depth函数求二叉树的高度, 并初始化res列表. 接下来是更新res的值, 使用递归, base case是当节点为空时, 返回res. 然后每次递归根据左边和右边的index来求中间的index, 节点放在中间. 如此递归.
Time: O(h*h), h为树的高度
Space: O(1)

代码

class Solution:
    def printTree(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[str]]
        """
        def height(root):
            if not root:
                return 0
            return 1 + max(height(root.left), height(root.right))
        
        def update(node, left, right, row, res):
            if not node:
                return 
            mid = (left + right)//2
            res[row][mid] = str(node.val)
            if node.left:
                update(node.left, left, mid-1, row+1, res)
            if node.right:
                update(node.right, mid+1, right, row+1, res)
                
        h = height(root)
        w = 2**h - 1
        res = [['']*w for i in range(h)]
        update(root, 0, w-1, 0, res)
        return res