Leetcode98 Validate 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.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

time O(n) space O(1)

class Solution {
public:
   int* last = NULL;
   bool isValidBST(TreeNode* root) {
       if (root){
           if(!isValidBST(root->left)) return false;
           if (last && *last>=root->val) return false;
           last = &root->val;
           if(!isValidBST(root->right)) return false;
           return true;
       }else return true;
   };
};