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
     */
    int pre = Integer.MIN_VALUE,cur = Integer.MIN_VALUE;
    public boolean isValidBST (TreeNode root) {
        // write code here
        if(root == null) return true;
        if(pre > cur) return false;
        if(isValidBST(root.left)) {
            pre = cur;
            cur = root.val;
        } 
        return pre <= cur &&isValidBST(root.right);
    }
    
}