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 TreeNode pre=null;
    public TreeNode head=null;
    public boolean pdfs(TreeNode root){
        if(root==null){
            return true;
        }
        if(!pdfs(root.left)){
            return false;
        }
        if(head == null){
            head = root;
            pre = root;
            // return true;
        }else{
            if (pre.val >= root.val){
                return false;
            }
            pre.right = root;
            // root.left = pre;
            pre =root;
        }
        if(!pdfs(root.right)){
            return false;
        }
        return true;
    }
    public boolean isValidBST (TreeNode root) {
        // write code here
        if(root ==null){
            return true;
        }
        if(!pdfs(root)){
            return false;
        }
        return true;
    }
}