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.
• A single node tree is a BST.

Solution: The in-order traversal of binary search tree is in ascending order. We only need to compare current value with the previous value. The previous value should be initialized with Long.MIN_VALUE, otherwise, this validation won't work with root.val = -2147483648.

Code:

public class Solution {

    private long lastValue = Long.MIN_VALUE;

    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (!isValidBST(root.left)) {
            return false;
        }
        if (root.val <= lastValue) {
            return false;
        }
        lastValue = root.val;
        return isValidBST(root.right);
    }
}