Q1.8 Check if s2 is a rotation of s1

Q: Assume you have a method isSubstring which checks if one word is asubstring of another. Given two strings, s1 and s2, write code tocheck if s2 is a rotation of s1 using only one call to isSubstring (i.e., “waterbottle” is a rotation of “erbottlewat”).

A:

s = xy, 其中x、y分别是子串， 旋转之后s' = yx , 可以发现无论从什么位置开始旋转， yx永远是xyxy是的子串。

#include <iostream>#include <string>using namespace std;bool isSubstring(string s, string t) {if (s.find(t) != string::npos) return true;else return false;}bool isRotation(string s1, string s2) {if (s1.length() != s2.length() || s1.length() < 1) {return false;}return isSubstring(s1+s1, s2);}int main() {string s1 = "waterbottle";    string s2 = "erbottlewat";    cout<<isRotation(s1, s2)<<endl;}