(leetcode) 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, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).

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]

使用回溯法：

class Solution {
public:
void backtrace(vector<int>& candidates,int sum,int target, vector<vector<int > > &results,vector<int> &result,int index)
{
if(sum == target)
{
results.push_back(result);
return;
}
else if(sum > target)
{
return;
}
else if(sum < target)
{
for(int i = index; i < candidates.size();++i)
{
result.push_back(candidates[i]);
backtrace(candidates, sum + candidates[i],target,results,result,i);
result.pop_back();
}
}

}
vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
sort(candidates.begin(),candidates.end());
vector<vector<int > > results;
vector<int> result;
int index = 0;
int sum = 0;

backtrace(candidates,sum,target,results,result,index);

return results;
}
};