leetcode 98 Valid Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).
Assume a 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:
1 | Input: |
Example 2:
1 | 5 |
- 使用一个stack,每遇到一个node,一直向左走,将经过的元素加入栈中
- pop出一个元素,使之为root,标记该root为pre,继续下一次向左走,此时,比较每一个经过的值和pre的大小
1 | class Solution { |