Validate Binary Search Tree

Desicription

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:

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2
3
  2
 / \
1   3

Binary tree [2,1,3], return true.

Example 2:

1
2
3
  1
 / \
2   3

Binary tree [1,2,3], return false.

Solution

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        return isValidBST(root, NULL, NULL);
    }
    bool isValidBST(TreeNode* root, TreeNode* minNode, TreeNode* maxNode) {
        if(!root)
            return 1;
        if((minNode && minNode->val >= root->val) || (maxNode && maxNode->val <= root->val))
            return 0;
        return isValidBST(root->left, minNode, root) && isValidBST(root->right, root, maxNode);
    }
};