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.

Example 2:

    1
   / \
  2   3

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

分析

递归传入最大值和最小值。用Long

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

    boolean helper(TreeNode root, long max, long min){
        if(root == null){
            return true;
        }
        if(root.val >= max || root.val <= min){
            return false;
        }
        return helper(root.left, root.val, min) && helper(root.right, max, root.val);
    }
}