Given two integers n and k, return all possible combinations of k numbers out of 1 ... n. http://www.leetcode.com/onlinejudge

leetcode_combinations.cpp

class Solution {
public:
    void dfs(vector<vector<int> >& ret, vector<int>& tmp, int n, int left, int k) {
        if (k == 0) {
            ret.push_back(tmp);
            return;
        }
        for (int i = left; i <= n; ++i) {
            tmp.push_back(i);
            dfs(ret, tmp, n, i + 1, k - 1);
            tmp.pop_back();
        }
    }

    vector<vector<int> > combine(int n, int k) {
        vector< vector<int> > ret;
        vector<int> tmp;
        dfs(ret, tmp, n, 1, k);
        return ret;
    }
};