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.

这题需要判断是不是一个正确的二叉搜索树，比较简单地一道题。

我们通过递归整棵树来解决，代码如下：

  1. class Solution {
  2. public:
  3.     bool isValidBST(TreeNode *root) {
  4.         return valid(root, numeric_limits<int>::min(), numeric_limits<int>::max());
  5.     }
  7.     bool valid(TreeNode* node, int minVal, int maxVal) {
  8.         if(!node) {
  9.             return true;
  10.         }
  12.         if(node->val <= minVal || node->val >= maxVal) {
  13.             return false;
  14.         }
  16.         return valid(node->left, minVal, node->val) &&
  17.         valid(node->right, node->val, maxVal);
  18.     }
  19. };