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验证二叉搜索树

Validate Binary Search Tree

专题
Algorithmic Programming / 算法编程
难度
L3
来源
Citadel

题目详情

问题：验证二叉搜索树

考察：树、递归

来源：Citadel

链接：https://www.jointaro.com/interviews/questions/validate-binary-search-tree/

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node's key.
  • The right subtree of a node contains only nodes with keys greater than the node's key.
  • Both the left and right subtrees must also be binary search trees.

 

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1
解析

思路：递归时为每个节点传入允许的开区间上下界，左子树必须小于当前值，右子树必须大于当前值。也可中序遍历检查序列严格递增。

复杂度：时间 O(n)，递归栈 O(h)。