2.Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n. You may return the answer in any order.

Example 1:
Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Example 2:
Input: n = 1, k = 1
Output: [[1]]

Solution: (Backtracking)

class Solution
{
public:
    vector<vector<int>> res;
    vector<int> a;

    void compute(int n, int k, int start)
    {

        if (k == 0)
        {
            res.push_back(a);
        }

        for (int i = start; i <= n; i++)
        {
            a.push_back(i);
            compute(n, k - 1, i + 1);
            a.pop_back();
        }
        return;
    }
    
    vector<vector<int>> combine(int n, int k)
    {

        compute(n, k, 1);
        return res;
    }
};

Time Complexity: O(n^k)