216. 组合总和 III

 1 class Solution {
 2 public:
 3     vector<vector<int>> combinationSum3(int k, int n) {
 4         back_tracking(k, n, 1, 0);
 5         return result;
 6     }
 7 private:
 8     vector<vector<int>> result;
 9     vector<int> tmp;
10     void back_tracking(int k, int n, int start, int sum){
11         if(tmp.size() == k && sum == n){
12             result.push_back(tmp);
13         }
14         for(int i = start; i <= 9; i++){
15             tmp.push_back(i);
16             back_tracking(k, n, i+1, sum+i);
17             tmp.pop_back();
18         }
19     } 
20 };
17. 电话号码的字母组合
 1 class Solution {
 2 private:
 3     vector<string> result;
 4     string path;
 5     const string vec[8] = {
 6         "abc",
 7         "def",
 8         "ghi",
 9         "jkl",
10         "mno",
11         "pqrs",
12         "tuv",
13         "wxyz"
14     };
15     void back_tracking(const string& digits, int start){
16         if(start == digits.size()){
17             result.push_back(path);
18             return;
19         }
20         int index = digits[start] - '2';
21         string str = vec[index];
22         for(int i = 0; i < str.size(); i++){
23             path.push_back(str[i]);
24             back_tracking(digits, start + 1);
25             path.pop_back();
26         }
27         
28     }
29 public:
30     vector<string> letterCombinations(string digits) {
31         if(digits.size() == 0)  return result;
32         back_tracking(digits, 0);
33         return result;
34     }
35 };