递归解法

#include <iostream>

using namespace std;
int d, n;

string solve(int n) {
  // recursion exit condition
  if (n == 1) return to_string(d);

  const string s = solve(n - 1);

  string ans;
  for (int i = 0, j = 0; j < s.length(); ++j) {
    if (j == s.length() - 1 || s[j] != s[j + 1]) {
      ans += s[j] + to_string(j - i + 1);
      i = j + 1;
    }
  }

  return ans;
}

int main(void) {
  scanf("%d %d", &d, &n);
  printf("%s\n", solve(n).c_str());
  return 0;
}