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:

    2
   / \
  1   3

Binary tree[2,1,3], return true.

代码

/**
 * 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 isBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
    }

    public boolean isBST(TreeNode root, long min, long max) {   //如果触碰边界，就是F 
        if (root == null) return true;
        if (root.val >= max || root.val <= min) return false;
        return isBST(root.left, min, root.val) && isBST(root.right, root.val, max);
    }
}