代码随想录算法训练营第37期 第二十一天 | LeetCode530.二叉搜索树的最小绝对差、501.二叉搜索树中的众数、236. 二叉树的最近公共祖先

---

一、530.二叉搜索树的最小绝对差

解题代码C++：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    int result = INT_MAX;
    TreeNode* pre = NULL;
    void traversal(TreeNode* cur)
    {
        if(cur == NULL) return;
        traversal(cur->left);
        if(pre != NULL)
            result = min(result, cur->val - pre->val);
        pre = cur;
        traversal(cur->right);
    }
public:
    int getMinimumDifference(TreeNode* root) {
        traversal(root);
        return result;
    }
};

题目链接/文章讲解/视频讲解：

https://programmercarl.com/0530.%E4%BA%8C%E5%8F%89%E6%90%9C%E7%B4%A2%E6%A0%91%E7%9A%84%E6%9C%80%E5%B0%8F%E7%BB%9D%E5%AF%B9%E5%B7%AE.html

---

---

二、501.二叉搜索树中的众数

解题代码C++：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
private:
    int maxCount = 0;
    int count = 0;
    TreeNode* pre = NULL;
    vector result;
    void searchBST(TreeNode* cur)
    {
        if(cur == NULL) return;
        
        searchBST(cur->left);
        if(pre == NULL)
            count = 1;
        else if(pre->val == cur->val)
            count ++;
        else
            count = 1;
        pre = cur;
        if(count == maxCount)
            result.push_back(cur->val);
        
        if(count > maxCount)
        {
            maxCount = count;
            result.clear();
            result.push_back(cur->val);
        }
        searchBST(cur->right);
        return;
    }
public:
    vector findMode(TreeNode* root) {
        count = 0;
        maxCount = 0;
        pre = NULL;
        result.clear();
        searchBST(root);
        return result;
    }
};

题目链接/文章讲解/视频讲解：

https://programmercarl.com/0501.%E4%BA%8C%E5%8F%89%E6%90%9C%E7%B4%A2%E6%A0%91%E4%B8%AD%E7%9A%84%E4%BC%97%E6%95%B0.html

---

---

三、236. 二叉树的最近公共祖先

解题代码C++：

/**
 * 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 == q || root == p || root == NULL) return root;
        TreeNode* left = lowestCommonAncestor(root->left, p, q);
        TreeNode* right = lowestCommonAncestor(root->right, p, q);
        if(left != NULL && right != NULL) return root;
        if(left == NULL) return right;
        return left;
    }
};

题目链接/文章讲解/视频讲解：

https://programmercarl.com/0236.%E4%BA%8C%E5%8F%89%E6%A0%91%E7%9A%84%E6%9C%80%E8%BF%91%E5%85%AC%E5%85%B1%E7%A5%96%E5%85%88.html