isValidBST created by Hexe - https://repl.it/@Hexe/isValidBST

isValidBST.cpp

#include <iostream>
#include <climits>

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};

class ValidateBST {
public:
    bool isValidBST(TreeNode* root) {
        return isValidBST(root, INT_MIN, INT_MAX);
    }

private:
    bool isValidBST(TreeNode* node, int lower, int upper) {
        if (node == nullptr) return true;
        int val = node->val;
        return val >= lower && val <= upper
               && isValidBST(node->left, lower, val)
               && isValidBST(node->right, val, upper);
    }
};

using namespace std;
int main() {

    auto root = new TreeNode(5);
    root->left = new TreeNode(3);
    root->right = new TreeNode(9);
    root->right->right = new TreeNode(10);


    ValidateBST solution = ValidateBST();
    cout << solution.isValidBST(root);
    return 0;
}