98. Validate Binary Search Tree

Posted 2020-12-30 tobeabetterpig

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
      return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }
    public boolean helper(TreeNode root, long minval, long maxval){
      if(root == null) return true;
      if(root.val >= maxval || root.val <= minval) return false;
      return helper(root.left, minval, root.val) && helper(root.right, root.val, maxval);
        
    }
}

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:

Input:
    2
   /   1   3
Output: true

Example 2:

    5
   /   1   4
     /     3   6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node‘s value
             is 5 but its right child‘s value is 4.