Validate Binary Search Tree 检测一个BST是否有效@LeetCode

思路：用上下限限定来递归

一开始想到了递归，但没想到用上下限的办法，而用多加一个parent作参数，结果非常麻烦。改成用上下限后代码很简洁清晰。

package Level3;import Utility.TreeNode;/** * 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.confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.OJ's Binary Tree Serialization:The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.Here's an example:   1  / \ 2   3    /   4    \     5The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}". * */public class S98 {public static void main(String[] args) {TreeNode root = new TreeNode(3);TreeNode p1 = new TreeNode(1);TreeNode p2 = new TreeNode(5);TreeNode p3 = new TreeNode(0);TreeNode p4 = new TreeNode(2);TreeNode p5 = new TreeNode(4);TreeNode p6 = new TreeNode(6);root.left = p1;root.right = p2;p1.left = p3;p1.right = p4;p2.left = p5;p2.right = p6;System.out.println(isValidBST(root));}public static boolean isValidBST(TreeNode root) {return rec(root, Integer.MIN_VALUE, Integer.MAX_VALUE);    }// 用最小值和最大值，来限定子树的范围public static boolean rec(TreeNode root, int min, int max){if(root == null){return true;}// 不在范围内if(root.val <= min || root.val >= max){return false;}// 检查左右子树的合法性并更新上下限return rec(root.left, min, root.val) && rec(root.right, root.val, max);}}

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isValidBST(TreeNode root) {        return isBSTRec(root, Integer.MIN_VALUE, Integer.MAX_VALUE);    }        public boolean isBSTRec(TreeNode root, int left, int right) {        if(root == null) {            return true;        }        if(root.val <= left || root.val >= right) {            return false;        }                return isBSTRec(root.left, left, root.val) && isBSTRec(root.right, root.val, right);    }}