Determine if a string s1 is a rotation of another string s2, by calling (only once) a function is_substring.

Constraints

• Can we assume the string is ASCII?
  • Yes
  • Note: Unicode strings could require special handling depending on your language
• Is this case sensitive?
  • Yes
• Can we use additional data structures?
  • Yes
• Can we assume this fits in memory?
  • Yes