中序遍历,从小到大
import java.util.*;
/*
* public class TreeNode {
* int val = 0;
* TreeNode left = null;
* TreeNode right = null;
* public TreeNode(int val) {
* this.val = val;
* }
* }
*/
public class Solution {
/**
* 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
*
*
* @param root TreeNode类
* @return bool布尔型
*/
public boolean isValidBST (TreeNode root) {
// write code here
// 中序遍历, 从小到大
if (root == null) return true;
LinkedList<TreeNode> st = new LinkedList<>();
TreeNode temp = root;
int pre = -Integer.MIN_VALUE;
while (!st.isEmpty() || temp != null) {
while (temp != null) {
st.push(temp);
temp = temp.left;
}
TreeNode t = st.poll();
int now = t.val;
if (pre > now) return false;
pre = now;
temp = t.right;
}
return true;
}
}