力扣（2024.08.12）
1. 98：验证二叉搜索树
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:def isValidBST(self, root: Optional[TreeNode]) -> bool:def dfs(node, res):if not node:returndfs(node.left, res)res.append(node.val)dfs(node.right, res)res = []dfs(root, res)for i in range(len(res) - 1):if res[i] >= res[i + 1]:return Falsereturn True2. 530：二叉搜索树的最小绝对差
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:def getMinimumDifference(self, root: Optional[TreeNode]) -> int:def dfs(node, res):if not node:returndfs(node.left, res)res.append(node.val)dfs(node.right, res)res = []dfs(root, res)final_res = res[-1]for i in range(len(res) - 1):if res[i + 1] - res[i] < final_res:final_res = res[i + 1] - res[i]return final_res3. 501：二叉搜索树中的众数
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:def findMode(self, root: Optional[TreeNode]) -> List[int]:def dfs(node, dic):if not node:returnif node.val in dic:dic[node.val] = dic[node.val] + 1else:dic[node.val] = 1dfs(node.left, dic)dfs(node.right, dic)dic = {}dfs(root, dic)final_res = []max_num = max(dic.values())for key in dic:if dic[key] == max_num:final_res.append(key)return final_res4. 108：将有序数组转换为二叉搜索树
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:def sortedArrayToBST(self, nums: List[int]) -> Optional[TreeNode]:def totree(nums, left, right):if left > right:returnmid = left + (right - left) // 2node = TreeNode(nums[mid])node.left = totree(nums, left, mid - 1)node.right = totree(nums, mid + 1, right)return noderoot = totree(nums, 0, len(nums) - 1)return root