DAY28

216组合总和-iii

未剪枝，注意i=startindex开始：

1. class Solution {
2. private:
3.     vector<vector<int>> result;
4.     vector<int> path;
5.     void backtracking(int k,int sum,int targetsum,int startindex){
6.         if(path.size()==k){
7.             if(sum==targetsum){
8.                 result.push_back(path);
9.             }
10.             return;
11.         }
12.         for(int i=startindex;i<=9;i++)
13.         {
14.             sum+=i;
15.             path.push_back(i);
16.             backtracking(k,sum,targetsum,i+1);
17.             sum-=i;
18.             path.pop_back();
19.         }
20.         return ;
21.     }
22. public:
23.     vector<vector<int>> combinationSum3(int k, int n) {
24.         result.clear();
25.         path.clear();
26.         backtracking(k,0,n,1);
27.         return result;
28.     }
29. };

剪枝版：

1. class Solution {
2. private:
3.     vector<vector<int>> result;
4.     vector<int> path;
5.     void backtracking(int k,int sum,int targetsum,int startindex){
6.         if(path.size()==k){
7.             if(sum==targetsum){
8.                 result.push_back(path);
9.             }
10.             return;
11.         }
12.         for(int i=startindex;i<=9-(k-path.size())+1;i++)
13.         {
14.             sum+=i;
15.             path.push_back(i);
16.             if(sum>targetsum){
17.             sum-=i;
18.             path.pop_back();
19.             return;
20.             }
21.             backtracking(k,sum,targetsum,i+1);
22.             sum-=i;
23.             path.pop_back();
24.         }
25.         return ;
26.     }
27. public:
28.     vector<vector<int>> combinationSum3(int k, int n) {
29.         result.clear();
30.         path.clear();
31.         backtracking(k,0,n,1);
32.         return result;
33.     }
34. };

17电话号码的字母组合

1. class Solution {
2. private:
3.     vector<string> map={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
4.     vector<string> result;
5.     string s;
6.     void backtracking(string digits,int index){
7.         //待二刷
8.         if(index==digits.size()){
9.             result.push_back(s);
10.             return ;
11.         }
12.         int digit=digits[index]-'0';
13.         string letters=map[digit];
14.         for(int i=0;i<letters.size();i++){
15.             s.push_back(letters[i]);
16.             backtracking(digits,index+1);
17.             s.pop_back();
18.         }
19.     }
20. public:
21.     vector<string> letterCombinations(string digits) {
22.         s.clear();
23.         result.clear();
24.         if(digits.size()==0){
25.             return result;
26.         }
27.         backtracking(digits,0);
28.         return result;
29.     }
30. };