Leetcode: Combinations

http://oj.leetcode.com/problems/combinations/

class Solution {
private:
    vector<bool> used;
    vector<vector<int> > res;
    vector<int> current;
public:
    // It asks for all the combinations, so we need the variable last to avoid duplications
    void DFS(int n, int k, int depth, int last=-1){
        if(depth==k){
            res.push_back(current);
        }
        else{
            for(int i=last+1;i<n;i++){
                if(used[i]==false){
                    current[depth]=i+1;
                    used[i]=true;
                    DFS(n, k, depth+1, i);
                    used[i]=false;
                }
            }
        }
    }
    vector<vector<int> > combine(int n, int k) {
        used.clear();
        res.clear();
        current.clear();
        if(k==0) return res;
        for(int i=0;i<n;i++) used.push_back(false);
        for(int i=0;i<k;i++) current.push_back(0);
        DFS(n, k, 0);
        return res;
    }
};