validate-binary-search-tree.cpp

// Time:  O(n)
// Space: O(h)

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    bool isValidBST(TreeNode *root) {
        if (!root) {
            return true;
        }

        if (!isValidBST(root->left)) {
            return false;
        }

        if (last && last != root && last->val >= root->val) {
            return false;
        }

        last = root;

        if (!isValidBST(root->right)) {
            return false;
        }

        return true;
    }

private:
    TreeNode *last = nullptr;
};