77. Combinations (Medium)

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],
]

Solution:

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