LeetCode 98 Validate Binary Search Tree

LeetCode 98

Validate Binary Search Tree

• Problem Description：

  判断给出的二叉树是否是二叉搜索树。

  二叉搜索树：要么是空树，要么是具有以下性质的二叉树：若它的左子树不为空，则左子树上所有结点的值均小于它的根节点的值；若它的右子树不空，则右子树上所有结点的值均大于它的根结点的值；它的左、右子树也分别为二叉搜索树。

  具体的题目信息：

  https://leetcode.com/problems/validate-binary-search-tree/description/

• Reference : https://blog.csdn.net/qq_30490125/article/details/53135274

• Solution:

  （1）方法一：根据定义递归实现

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        if (root == NULL)
            return true;
        if (root->left && maxNum(root->left)>= root->val)
            return false;
        if (root->right && minNum(root->right)<= root->val)
            return false;
        if (!(isValidBST(root->left)&&isValidBST(root->right)))
            return false;
        return true;
    }
    int maxNum(TreeNode* p) {
        while(p->right) {
            p = p->right;
        }
        return p->val;
    }
    int minNum(TreeNode* p) {
        while(p->left) {
            p = p->left;
        }
        return p->val;
    }
};
           

（2） 方法二：利用中序遍历把所有结点的值存储在vector数组中，判断数组中元素是否递增排列。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        if (root == NULL) return true;
        stack<TreeNode*> node;
        vector<int> num;
        TreeNode* p = root;
        while(p || !node.empty()) {
            while(p) {
                node.push(p);
                p = p->left;
            }
            p = node.top();
            num.push_back(p->val);
            node.pop();
            p = p->right;
        }
        for (int i = ; i < num.size()-; i++) {
            if (num[i]>=num[i+])
                return false;
        }
        return true;
    }
};