题目描述：

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] 

代码：

void get_combination(vector<int> vec,int index,vector<int> candidates,int target,vector<vector<int> > & result)
{
    if(target == 0)
    {
        result.push_back(vec);
        return;
    }
    for(int i = index;i < candidates.size();i++)
    {
        if(target - candidates[i] < 0)
            continue;
        vector<int> temp = vec;
        temp.push_back(candidates[i]);
        get_combination(temp,i,candidates,target-candidates[i],result);
    }
}

vector<vector<int> > Solution::combinationSum(vector<int> &candidates, int target)
{
    vector<vector<int> > result;
    sort(candidates.begin(),candidates.end());
    vector<int> vec;
    get_combination(vec,0,candidates,target,result);
    return result;
}

LeetCode:Combination Sum