代码随想录算法训练营第36期DAY28

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. };