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:

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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);
}
};