Question

I have two strings a and b. I want to know if a is rotation of b or vice versa without creating a third string.

Answer 1

A string a is a rotation of b if and only if there is an L such that L == len(a) == len(b), and there is an offset 0 <= j < len(a) such that a[ (i+j) % L] == b[i] for all 0 <= i < L.

In c code (assuming that L is the common length of the strings. Returns 1 if a is a rotation of b, 0 otherwise):

int i, j, is_rot;
for (i = 0; i< L; i++){
    is_rot = 1;
    for (j = 0; j<L; j++){
        if (a[ (j + i) %L] != b[j] ){
            is_rot = 0;
            break;
        }
    }
    if (is_rot) return 1;
}
return 0;