C++第二阶段——数据结构和算法，之前学过一点点数据结构，当时是基于Python来学习的，现在基于C++查漏补缺，尤其是树的部分。这一部分计划一个月，主要利用代码随想录来学习，刷题使用力扣网站，不定时更新，欢迎关注！

一、77. 组合

class Solution {
public:vector<vector<int>> result;vector<int> path;vector<vector<int>> combine(int n, int k) {insertVector(n,k,1);return result;}void insertVector(int n,int k,int startIndex){if(path.size()==k){result.push_back(path);return;}for(int i=startIndex;i<=n;i++){path.push_back(i);insertVector(n,k,i+1);path.pop_back();}}
};

二、216. 组合总和 III

class Solution {
public:int targetSum;vector<int> path;vector<vector<int>> result;vector<vector<int>> combinationSum3(int k, int n) {insertVector(k,n,1);return result;}void insertVector(int k,int n,int startIndex){if(path.size()==k){if(targetSum==n){result.push_back(path);return;}}for(int i=startIndex;i<=9;i++){path.push_back(i);targetSum+=i;insertVector(k,n,i+1);path.pop_back();targetSum-=i;}}
};

三、17. 电话号码的字母组合

class Solution {
public:vector<string> result;string s="";string stringV[10] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};vector<string> letterCombinations(string digits) {insertString(digits,0);return result;}void insertString(string digits,int index){if(index==digits.size()){// 收获结果if(s==""){return;}result.push_back(s);return ;}int digit = digits[index]-'0';string nextS = stringV[digit];// 遍历for(int i=0;i<nextS.size();i++){s+= nextS[i];insertString(digits,index+1);s.pop_back();}}
};

四、39. 组合总和

class Solution {
public:vector<vector<int>> result;vector<int> path;int Asum =0;vector<vector<int>> combinationSum(vector<int>& candidates, int target) {insertV(candidates,target,0);return result;}void insertV(vector<int> candidates,int target,int startIndex){if(Asum==target){// 收获结果result.push_back(path);return;}else if(Asum>target){return;}for(int i=startIndex;i<candidates.size();i++){path.push_back(candidates[i]);Asum+=candidates[i];insertV(candidates,target,i);// 回溯path.pop_back();Asum-=candidates[i];}}
};

五、40. 组合总和 II

class Solution {
public:vector<vector<int>> result;vector<int> path;int Asum;vector<vector<int>> combinationSum2(vector<int>& candidates, int target) {// 首先进行排序sort(candidates.begin(),candidates.end());vector<bool> used(candidates.size(),false); // 和candidates等长，并且全为0；insertS(candidates,target,0,used);return result;}void insertS(vector<int> candidates,int target,int startIndex,vector<bool> used){if(Asum==target){result.push_back(path);return;}if(Asum>target){return;}// 用一个used数组表示是否使用过for(int i=startIndex;i<candidates.size();i++){if (i > 0 && candidates[i] == candidates[i - 1] && used[i - 1] == false) {continue;}path.push_back(candidates[i]);Asum+= candidates[i];used[i] = true;insertS(candidates,target,i+1,used);used[i] =false;path.pop_back();Asum-= candidates[i];}}
};