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.

设置一个最大和最小值，同时更新。

 public class Solution {
    //Time: O(n)
    public boolean isValidBST(TreeNode root) {
        return isValid(root, Integer.MAX_VALUE, Integer.MIN_VALUE);
    }
    
    private boolean isValid(TreeNode root, int max, int min) {
        if (root == null) {
            return true;
        }
        
        if (root.val >= max || root.val <= min) {
            return false;
        }
        return isValid(root.left, root.val, min) && isValid(root.right, max, root.val);
    }
}

用类似一个inorder的方法，灵感来自recover BST。

public class Solution {
    private TreeNode last = null;
    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }
        
        boolean left = isValidBST(root.left);
        if (last != null && last.val >= root.val) {
            return false;
        }
        last = root;
        boolean right = isValidBST(root.right);
        return left && right;
    }
}