ValidateBinarySearchTree.java

package com.stevesun.solutions;

import com.stevesun.common.classes.TreeNode;

/**
 * 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:
   2
  / \
 1   3
 Binary tree [2,1,3], return true.

 Example 2:
   1
  / \
 2   3
 Binary tree [1,2,3], return false.
 */
public class ValidateBinarySearchTree {
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        return dfs(root.left, Long.MIN_VALUE, root.val) && dfs(root.right, root.val, Long.MAX_VALUE);

    }

    private boolean dfs(TreeNode root, long minValue, long maxValue) {
        if (root == null) return true;
        if (root != null && (root.val <= minValue || root.val >= maxValue)) return false;
        boolean leftResult = true, rightResult = true;
        if (root.left != null) leftResult = dfs(root.left, minValue, root.val);
        if (root.right != null) rightResult = dfs(root.right, root.val, maxValue);
        return leftResult && rightResult;
    }
}