力扣hot100 - 98、验证二叉搜索树
题目：
思路：二叉搜索树如果按照中序遍历，那么遍历的结果一定是递增的。我们那一个pre指针指向遍历元素的前一个元素，不断比较更新当前元素和前一个元素的值。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode() {}
* TreeNode(int val) { this.val = val; }
* TreeNode(int val, TreeNode left, TreeNode right) {
* this.val = val;
* this.left = left;
* this.right = right;
* }
* }
*/
class Solution {
public TreeNode pre = null;
public boolean isValidBST(TreeNode root) {
if(root == null) return true;
boolean l = isValidBST(root.left);
if(pre != null && root.val <= pre.val){
return false;
}
pre = root;
boolean r = isValidBST(root.right);
return l && r;
}
}
