方法一：二叉树的中序遍历
/**
* 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 boolean isValidBST(TreeNode root) {
//二叉树有一个特点在于它的中序遍历结果升序排列
//定义一个栈用于控制中序遍历的顺序
Stack<TreeNode> stack = new Stack<>();
//定义起始节点
TreeNode cur = root;
long pre = Long.MIN_VALUE;
//循环中序遍历
while(!stack.isEmpty() || cur != null){
//首先将本节点的左节点全部入栈
while(cur != null){
stack.push(cur);
cur = cur.left;
}
//出栈第一个节点
cur = stack.pop();
//对节点判断是否符合条件
if(pre >= cur.val){
return false;
}
pre = cur.val;
//继续判断右子树
cur = cur.right;
}
return true;
}
}
方法二：逐个递归验证
/**
* 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 boolean isValidBST(TreeNode root) {
return check(root,Long.MIN_VALUE,Long.MAX_VALUE);
}
private boolean check(TreeNode root,long left,long right){
//设置递归出口
if(root == null){
return true;
}
//判断当前值是否满足条件
if(left >= root.val || root.val >= right){
return false;
}
//递归遍历左右子树是否满足条件,遍历左子树时将当前的节点的值作为右边界，遍历右子树时减当前节点作为左边界
return check(root.left,left,(long)root.val) && check(root.right,(long)root.val,right);
}
}