[LeetCode] Combinations

Problem Statement (link):

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Analysis:
Another classic DFS problem. Once we see "all possible" key words, we need to consider using DFS idea.

Two things to think about:
1) Avoid duplicates - we use a int st to avoid choosing the same combination again. In other words, we only choose the numbers larger than current numbers.
2) Choose k numbers - we decrease k each time we choose a number.

Code:

class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
        vector<vector<int> > outv;
        if (k==0 || n<k) return outv;
        vector<int> res;
        recur(outv, res, n, k, 0);
        return outv;
    }

    void recur(vector<vector<int>> &outv, vector<int> &res, int n, int k, int st) {
        if (k==0) {
            outv.push_back(res);
            return;
        }

        for (int i=st; i<n; i++) {
            res.push_back(i+1);
            recur(outv, res, n, k-1, i+1);
            res.pop_back();
        }
        return;
    }
};