Combination Sum

Given a set of candidate numbers (C) and a target number (T), find all unique combinations inC 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]

Solution:

class Solution {public:    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {        vector<vector<int> > res;        if(!candidates.size()) return res;        vector<int> v;        sort(candidates.begin(), candidates.end());        dfs(candidates, target, 0, v, res);                return res;    }        void dfs(vector<int> &candidates, int target, int num, vector<int> &v, vector<vector<int> > &res)    {        if(num > target) return ;        if(num == target)        {            vector<int> tmp(v.size());            for(int i = 0; i < v.size(); ++i) tmp[i] = candidates[v[i]];            res.push_back(tmp);            return ;        }        int i = v.empty() ? 0 : v[v.size() - 1];        for(; i < candidates.size(); ++i)        {            v.push_back(i);            dfs(candidates, target, num + candidates[i], v, res);            v.pop_back();        }    }};