Leetcode OJ 98 Validate Binary Search Tree [Medium]
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Leetcode OJ 98 Validate Binary Search Tree
题目描述:
Given a binary tree, determine if it is a valid binarysearch tree (BST).
Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keysless than the node's key.
The right subtree of a node contains only nodes withkeys greater than the node's key.
Both the left and right subtrees must also be binarysearch 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.
题目理解:
判断一个二叉树是不是二叉查找树。
测试用例:
功能测试:[2,1,3];[10,5,15,null,null,6,20](父节点介于左节点和右节点间,但不是二叉搜索树);
特殊输入:二叉树是空;二叉树的节点是int边界值;
分析:
1. 一开始想到只要父节点介于左右节点之间就是二叉搜索树,但这是不对的比如[10,5,15,null,null,6,20];
2. 左节点小于父节点,大于最小值,最小值是继承下来的;
3. 右节点大于父节点,小于最大值,最大值是继承下来的;
4. 如果出现左节点大于等于父节点,或者小于等于最小值,则返回false;
5. 如果出现右节点小于等于父节点,或者大于等于最大值,则返回false;
6. 用递归实现,递归的边界是要检查的节点是null;
7. 最初设置的最小最大值要比int的边界更小/大;比如当输入[int.MAX_VALUE]时,返回值应是true;
解答:
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */public class Solution { public boolean isValidBST(TreeNode root) { return isValidBSTNode(root,Long.MIN_VALUE,Long.MAX_VALUE); } public static boolean isValidBSTNode(TreeNode root,long minValue,long maxValue){ if(root== null) returntrue; if(root.val<= minValue || root.val >= maxValue) return false; return isValidBSTNode(root.left, minValue, root.val) && isValidBSTNode(root.right,root.val, maxValue); }}
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