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 max = 10000;
    boolean flag = true;
    public boolean isValidBST (TreeNode root) {
        // write code here
        if (root == null || (root.left == null && root.right == null)) {
            return true;
        }

        if (!IsSuitable(root, root.val)) {
            return false;
        }

        if ((root.left == null || root.left.val < root.val) &&
                (root.right == null || root.right.val > root.val)) {
            return isValidBST(root.left) && isValidBST(root.right);
        }
        return false;

    }

    public boolean IsSuitable(TreeNode node, int num) {
        TreeNode temp = node;
        boolean flag1 = false;
        boolean flag2 = false;
        node = node.left;
        if (node == null) {
            flag1 = true;
        } else {
            if (node.right == null) {
                flag1 = true;
            } else {
                while (node.right.right != null) {
                    node = node.right;
                }
                flag1 = node.right.val < num;
            }
        }

        node = temp.right;
        if (node == null) {
            flag2 = true;
        } else {
            if (node.left == null) {
                flag2 = true;
            } else {
                while (node.left.left != null) {
                    node = node.left;
                }
                flag2 = node.left.val > num;
            }
        }

        return flag1 && flag2;
    }
}