text: // Time:  O(1)
  // Space: O(1)
  
  class Solution {
  public:
      int countVowelStrings(int n) {
          return nCr(n + 4, 4);  // H(5, n) = C(n+5-1, n) = C(n+4, 4)
      }
  
  private:
      uint64_t nCr(int n, int r) {  // Time: O(n), Space: O(1)
          if (n - r < r) {
              return nCr(n, n - r);
          }
          uint64_t c = 1;
          for (int k = 1; k <= r; ++k) {
              c *= n - k + 1;
              c /= k;
          }
          return c;
      }
  };
  