77. Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

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

AC code:

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

Runtime: 88 ms, faster than 42.24% of C++ online submissions for Combinations.