LeetCode 1372. Longest ZigZag Path in a Binary Tree二叉树中的最长交错路径【Medium】【Python】【DFS】

Problem

LeetCode

Given a binary tree root, a ZigZag path for a binary tree is defined as follow:

  • Choose any node in the binary tree and a direction (right or left).
  • If the current direction is right then move to the right child of the current node otherwise move to the left child.
  • Change the direction from right to left or right to left.
  • Repeat the second and third step until you can't move in the tree.

Zigzag length is defined as the number of nodes visited - 1. (A single node has a length of 0).

Return the longest ZigZag path contained in that tree.

Example 1:

Input: root = [1,null,1,1,1,null,null,1,1,null,1,null,null,null,1,null,1]
Output: 3
Explanation: Longest ZigZag path in blue nodes (right -> left -> right).

Example 2:

Input: root = [1,1,1,null,1,null,null,1,1,null,1]
Output: 4
Explanation: Longest ZigZag path in blue nodes (left -> right -> left -> right).

Example 3:

Input: root = [1]
Output: 0

Constraints:

  • Each tree has at most 50000 nodes..
  • Each node's value is between [1, 100].

问题

力扣

给你一棵以 root 为根的二叉树,二叉树中的交错路径定义如下:

  • 选择二叉树中 任意 节点和一个方向(左或者右)。
  • 如果前进方向为右,那么移动到当前节点的的右子节点,否则移动到它的左子节点。
  • 改变前进方向:左变右或者右变左。
  • 重复第二步和第三步,直到你在树中无法继续移动。

交错路径的长度定义为:访问过的节点数目 - 1(单个节点的路径长度为 0 )。

请你返回给定树中最长 交错路径 的长度。

示例 1:

输入:root = [1,null,1,1,1,null,null,1,1,null,1,null,null,null,1,null,1]
输出:3
解释:蓝色节点为树中最长交错路径(右 -> 左 -> 右)。

示例 2:

输入:root = [1,1,1,null,1,null,null,1,1,null,1]
输出:4
解释:蓝色节点为树中最长交错路径(左 -> 右 -> 左 -> 右)。

示例 3:

输入:root = [1]
输出:0

提示:

  • 每棵树最多有 50000 个节点。
  • 每个节点的值在 [1, 100] 之间。

思路

DFS

递归时记录前面的节点是左节点还是右节点,以及深度。
Python3代码
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def longestZigZag(self, root: TreeNode) -> int:
        if root == None:
            return 0
        self.max_ = 0
        self.dfs(root, 0, 0)
        return self.max_

    def dfs(self, root, prev, depth):
        self.max_ = max(depth, self.max_)

        if root.left:
            # left->left
            if prev == 0:
                self.dfs(root.left, 0, 1)
            # left->right
            else:
                self.dfs(root.left, 0, depth + 1)
        if root.right:
            # right->right
            if prev == 1:
                self.dfs(root.right, 1, 1)
            # right->left
            else:
                self.dfs(root.right, 1, depth + 1)

代码地址

GitHub链接