Level 1 (day 8)

第一题

题目链接：https://leetcode.cn/problems/validate-binary-search-tree/

个人题解：中序遍历数组有序为BST

代码：

class Solution {
public:
    bool isValidBST(TreeNode* root) {
        vector<int> ans;
        inorder(root,ans);
        for(int i=0;i<ans.size()-1;i++)
        {
            if(ans[i]>=ans[i+1]) return false;
        }
        return true;
    }
    void inorder(TreeNode* root,vector<int>& ans)
    {
        if(!root) return;
        inorder(root->left,ans);
        ans.push_back(root->val);
        inorder(root->right,ans);
    }
};

第二题

题目链接：https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-search-tree/

个人题解：递归

代码：

/**
 * 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:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root==NULL) return NULL;//空返回空
        if(root->val==p->val||root->val==q->val) return root;// 示例二
        if(root->val<p->val&&root->val<q->val) return lowestCommonAncestor(root->right,p,q);//比他小，向右边
        else if(root->val>p->val&&root->val>q->val) return lowestCommonAncestor(root->left,p,q);//比他大，向左边
        else return root;
    }
};