216.组合总和III

class Solution {
public:vector<vector<int>> result;vector<int> temp;void backtracking(int begin, int k, int n, int sum){if(temp.size()==k){if(sum==n){result.push_back(temp);}return;}for(int i = begin; i<=9-(k-temp.size())+1; i++){if(n-sum<i){return;}temp.push_back(i);sum += i;backtracking(i+1, k, n, sum);temp.pop_back();sum -= i;// k++;}}vector<vector<int>> combinationSum3(int k, int n) {if(n>45 || k<=0){return result;}int sum = 0;backtracking(1, k, n, sum);return result;}
};

参考文章：代码随想录-216.组合总和III

17.电话号码的字母组合

class Solution {
private:const string lettermap[10] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
public:vector<string> result;string temp;void backtracking(string& digits, int index){if(temp.size() == digits.size()){result.push_back(temp);return;}int digit = digits[index]-'0';string letters = lettermap[digit];for(int i=0; i<letters.size(); i++){temp.push_back(letters[i]);backtracking(digits, index+1);temp.pop_back();}}vector<string> letterCombinations(string digits) {if(digits.size()==0){return result;}backtracking(digits, 0);return result;}
};

参考文章：代码随想录-17.电话号码的字母组合