题解 | #判断是不是二叉搜索树#

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
        LinkedList<TreeNode> stack=new LinkedList<>();
        TreeNode node=root;
        TreeNode pop=null;
        while(node!=null||!stack.isEmpty()){
            if(node!=null){
                stack.push(node);
                node=node.left;
            }else{
                pop=stack.pop();
                if(stack.peek()!=null&&pop.val>stack.peek().val){
                    return false;
                }
                node=pop.right;
            }
        }
        return true;
    }

    
}