算法训练营day23补签

 题目1：530. 二叉搜索树的最小绝对差 - 力扣（LeetCode）

class Solution {
public:
    int reslut = INT_MAX;
    TreeNode* pre = NULL;
    void trackingback(TreeNode* node) {
        if(node == NULL) return;
        trackingback(node->left);
        if(pre != NULL) {
            reslut = min(reslut, node->val - pre->val);
        }
        pre = node;
        trackingback(node->right);
    }  
    int getMinimumDifference(TreeNode* root) {
        trackingback(root);
        return reslut;
    }
};

 题目2：501. 二叉搜索树中的众数 - 力扣（LeetCode）

// 中序遍历用map记录每个数出现的个数，然后找最大的
class Solution {
public:
    unordered_map<int, int> map;
    void trackingback(TreeNode* node) {
        if(node == NULL) return;
        trackingback(node->left);
        if(map.find(node->val) != map.end()) {
            map[node->val]++;
        }else {
            map.insert(pair<int, int>(node->val, 1));
        }
        trackingback(node->right);
    }
    vector<int> findMode(TreeNode* root) {
        vector<int> reslut;
        trackingback(root);
        int num = 1;
        for(auto iter =map.begin();iter != map.end();iter++) {
            if(iter->second > num) {
                num = iter->second;
            }
        }
        for(auto iter =map.begin();iter != map.end();iter++) {
            if(iter->second == num) {
                reslut.push_back(iter->first);
            }
        }
        return reslut;
    }
};

在递归里操作，主要就是更新maxcount 然后要把reslut也更新 

class Solution {
public:
    vector<int> reslut;
    int count = 0;
    int maxcount = 0;
    TreeNode* pre = NULL;
    void trackingback(TreeNode* node) {
        if(node == NULL) return;
        trackingback(node->left);
        if(pre == NULL) {
            count = 1;
        }else if(pre->val == node->val) {
            count++ ;
        }else if(pre->val != node->val) {
            count = 1;
        }
        pre = node;
        if(count == maxcount) {
            reslut.push_back(node->val);
        }

        if(count > maxcount) {
            reslut.clear();
            maxcount = count;
            reslut.push_back(node->val);
        }
        trackingback(node->right);
    }
    vector<int> findMode(TreeNode* root) {
        trackingback(root);
        return reslut;
    }
};

 题目3：236. 二叉树的最近公共祖先 - 力扣（LeetCode）

class Solution {
public:
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        if(root == NULL) return NULL;
        if(root == p || root == q) return root;
        TreeNode* leftnode = lowestCommonAncestor(root->left, p, q);
        TreeNode* rightnode = lowestCommonAncestor(root->right, p, q);
        if(rightnode != NULL && leftnode != NULL) return root;
        else if(rightnode == NULL && leftnode != NULL) return leftnode;
        else if(rightnode != NULL && leftnode == NULL) return rightnode;
        else return NULL;
    }
};