98. Validate Binary Search Tree7. Reverse Integer.cpp

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:

    2
   / \
  1   3

Input: [2,1,3]
Output: true
Example 2:

    5
   / \
  1   4
     / \
    3   6

Input: [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.



/**
 * 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) {}
 * };
 */

bool tmpIsValidBST(TreeNode *root, long long mini, long long maxi){
    if(!root) return true;
    if(root->val<=mini || root->val>=maxi) return false;
    return tmpIsValidBST(root->left, mini, root->val) && tmpIsValidBST(root->right, root->val, maxi);

}

class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return tmpIsValidBST(root, (long long)INT_MIN-1, (long long)INT_MAX+1);
    }
};