【leetcode】Combination Sum

Combination Sum

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a₁, a₂, … , a_k) must be in non-descending order. (ie, a₁ ≤ a₂ ≤ … ≤ a_k).
• The solution set must not contain duplicate combinations.

For example, given candidate set 2,3,6,7 and target 7, 
A solution set is: 
[7] 
[2, 2, 3] 

利用递归搜索，同时剪枝就可以

 

 1 class Solution {

 2 

 3 public:

 4 

 5     vector<vector<int> > combinationSum(vector<int> &candidates, int target) {

 6 

 7         

 8 

 9         sort(candidates.begin(),candidates.end());

10 

11         vector<vector<int> > result;

12 

13         vector<int> tmp;

14 

15  

16 

17         backtracking(result,candidates,target,tmp,0,0);

18 

19         

20 

21         return result;

22 

23     }

24 

25     

26 

27     void backtracking(vector<vector<int> > &result,vector<int> &candidates, int &target,vector<int> tmp, int sum,int index)

28 

29     {

30 

31  

32 

33         if(sum==target)

34 

35         {

36 

37             result.push_back(tmp);

38 

39             return;

40 

41         }

42 

43         else

44 

45         {

46 

47             int sum0=sum;

48 

49             //注意index

50 

51             for(int i=index;i<candidates.size();i++)

52 

53             {

54 

55                 tmp.push_back(candidates[i]);

56 

57                 sum=sum0+candidates[i];

58 

59                 if(sum>target)

60 

61                 {

62 

63                     break;

64 

65                 }

66 

67                 backtracking(result,candidates,target,tmp,sum,i);

68 

69                 tmp.pop_back();

70 

71             }            

72 

73         }        

74 

75     }

76 

77 };