Solution.cpp

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> ans;
        vector<int> t;
        function<void(int)> dfs = [&](int i) {
            if (t.size() == k) {
                ans.emplace_back(t);
                return;
            }
            if (i > n) {
                return;
            }
            t.emplace_back(i);
            dfs(i + 1);
            t.pop_back();
            dfs(i + 1);
        };
        dfs(1);
        return ans;
    }
};