Today I have done leetcode's August LeetCoding Challenge with cpp.

August LeetCoding Challenge 1

Description

Combinations

Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: There are 4 choose 2 = 6 total combinations.
Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.

Example 2:

Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.

Constraints:

• 1 <= n <= 20
• 1 <= k <= n

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
public:
  vector<vector<int>> combine(int n, int k) {
    vector<vector<int>> answer;
    for(int mask = (1 << k) - 1; mask < (1 << n); mask = (mask + (mask & -mask)) | ((mask & ~(mask + (mask & -mask))) / (mask & -mask) >> 1)) {
      vector<int> result;
      for(int i = 0; i < n; ++i) {
        if(mask & (1 << i)) {
          result.push_back(i + 1);
        }
      }
      answer.push_back(result);
    }
    return answer;
  }
};

// Accepted
// 27/27 cases passed (11 ms)
// Your runtime beats 95.34 % of cpp submissions
// Your memory usage beats 40.55 % of cpp submissions (10.6 MB)