static int maxRunlengthOfRepeatingChars(S s) {
  if (empty(s)) ret 0;
  int max = 0;
  for (int i = 0; i < l(s); ) {
    char c = s.charAt(i);
    int j = i+1;
    while (j < l(s) && s.charAt(j) == c) ++j;
    int n = j-i;
    max = max(max, j-i);
    i = j;
  }
  ret max;
}