98 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.

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.

Recursion

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private boolean isValid;
    private TreeNode prev;
    public boolean isValidBST(TreeNode root) {
        isValid = true;
        prev = null;
        if (root == null) {
            return true;
        }
        helper(root);
        return isValid;
    }
    public void helper(TreeNode root) {
        if (root == null) {
            return;
        }
        helper(root.left);
        if (prev != null && prev.val >= root.val) { // [1,1]->should return false;
            isValid = false;
            return;
        }
        prev = root;
        helper(root.right);
    }
}