1. use recursion:

https://leetcode.com/problems/validate-binary-search-tree/

  1. /**
  2.  * Definition for a binary tree node.
  3.  * struct TreeNode {
  4.  *     int val;
  5.  *     TreeNode *left;
  6.  *     TreeNode *right;
  7.  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  8.  * };
  9.  */
  10. class Solution {
  11. public:
  12.     bool isValidBST(TreeNode* root) {
  13.         return isValidBST(root, LONG_MIN, LONG_MAX);
  14.     }
  16.     bool isValidBST(TreeNode* root, long lower, long upper) {
  17.         if (!root) return true;
  19.         if(root->val <= lower || root->val >= upper)
  20.             return false;
  22.         return isValidBST(root->left, lower, root->val)
  23.             && isValidBST(root->right, root->val, upper);
  24.     }
  25. };