Given two strings s and goal, return true if and only if s can become goal after some number of left rotations — repeatedly moving characters from the front of the string to the back (e.g. rotating "abcde" by 2 gives "cdeab"). Trying every rotation offset and rebuilding the string each time works, but it's wasteful. The key trick: every rotation of s is guaranteed to appear as a contiguous substringSubstringA contiguous run of characters taken from within a larger string — unlike a subsequence, the characters must be adjacent and in order. somewhere inside s + s. So the whole problem reduces to one substring-containment check on the doubled string.