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] 

class Solution {public:    void visit(vector<int> &candidates, int n, int pos, int target, int sum,                 vector<vector<int> > &result, vector<int> &buf)    {    if (sum == target)    {    result.push_back(buf);    return;    }    if (pos >= n)    {    return;    }    int i =  0;    while (true)    {    int temp = sum + candidates[pos]*i;    if (temp > target)    {    break;    }    else    {    for (int j = 0; j < i; j++)    {    buf.push_back(candidates[pos]);    }    visit(candidates, n, pos+1, target, temp, result, buf);    for (int j = 0; j < i; j++)    {    buf.pop_back();    }    }    i++;    }    }    vector<vector<int> > combinationSum(vector<int> &candidates, int target) {        int n = candidates.size();    sort(candidates.begin(), candidates.end());    vector<vector<int> > result;    vector<int> buf;    int sum = 0;     visit(candidates, n, 0, target, sum, result, buf);    return result;      }};