题解 | #判断是不是二叉搜索树#

using System;
using System.Collections.Generic;

/*
public class TreeNode
{
    public int val;
    public TreeNode left;
    public TreeNode right;

    public TreeNode (int x)
    {
        val = x;
    }
}
*/

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 bool isValidBST (TreeNode root) {
        // write code here
        if (root == null) return false;
        if (root.left == null && root.right == null) return true;
        if (root.left != null && root.right == null) {
            return root.left.val < root.val && isValidBST(root.left);
        }
        if (root.right != null && root.left == null) {
            return root.right.val > root.val && isValidBST(root.right);
        }
        if (root.left.val < root.val && root.right.val > root.val) {
            if (root.left.right != null && root.right.left == null) {
                return root.val > root.left.right.val && isValidBST(root.left) &&
                       isValidBST(root.right);
            } else if (root.left.right == null && root.right.left != null)
                return root.val < root.right.left.val && isValidBST(root.left) &&
                       isValidBST(root.right);
            else if (root.left.right == null && root.right.left == null)
                return isValidBST(root.left) && isValidBST(root.right);
            else
                return root.val > root.left.right.val && root.val < root.right.left.val &&
                       isValidBST(root.left) && isValidBST(root.right);
        }
        return false;
    }
}