【LeetCode】98. Validate Binary Search Tree(Medium)解题报告

【LeetCode】98. Validate Binary Search Tree(Medium)解题报告

题目地址：https://leetcode.com/problems/validate-binary-search-tree/description/

题目描述：

  Given a binary tree, determine if it is a valid binary search tree (BST).

  Assume a BST is defined as follows:

    1. The left subtree of a node contains only nodes with keys less than the node’s key.

    2. The right subtree of a node contains only nodes with keys greater than the node’s key.

    3. 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.

  判断是否是二叉搜索树。

Solution:

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
time : O(n)
space : O(n)
*/
class Solution {
public boolean isValidBST(TreeNode root) {
if(root == null) return true;
return helper(root,null,null);
}
public static boolean helper(TreeNode root,Integer min,Integer max){
if(root == null) return true;
if(min != null && root.val <= min) return false;
if(max != null && root.val >= max) return false;
return helper(root.left,min,root.val) && helper(root.right,root.val,max);
}
}

Date：2018年3月11日