private boolean isSubstring(String s1s2, String s2) {
    return s1s2.contains(s2);
  }
Iterative algorithm used to solve this problem. The complexity order of this algorithm in time
and space terms is O(N) where N is the sum of input strings lengths. To solve this problem we
check if the length of input strings is the same and for strings with the same length we are
going to concatenate s1 with s1 check if duplicated s1 contains s2.

s1 = waterbottle
s2 = erbotlewat
s1s1 = waterbottlewaterbottle
return "waterbottlewaterbottle".contains(erbotlewat)
src/main/java/com/github/pedrovgs/problem56/IsRotationUsingIsSubstring.java