class Solution {
private:vector<vector<int>> result;vector<int> path;void backtracking(int targetSum, int k, int sum, int startIndex) {if (path.size() == k) {if (sum == targetSum) result.push_back(path);}for (int i = startIndex; i <= 9; i++) {sum += i;path.push_back(i);backtracking(targetSum, k, sum, i + 1);sum -= i;path.pop_back();}}
public:vector<vector<int>> combinationSum3(int k, int n) {result.clear();path.clear();backtracking(n, k, 0, 1);return result;}
};

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 s;void backtracking(const string& digits, int index) {if (index == digits.size()) {result.push_back(s);return;}int digit = digits[index] - '0';string letters = letterMap[digit];for (int i = 0; i < letters.size(); i++) {s.push_back(letters[i]);backtracking(digits, index + 1);s.pop_back();}}vector<string> letterCombinations(string digits) {s.clear();result.clear();if (digits.size() == 0) {return result;}backtracking(digits, 0);return result;}
};