Find Next Character in Sorted String

Question:
Given a sorted array of unique elements R, that are letters of English alphabet and an input character x. The elements in R are sorted with the least element appearing first. Find the minimum r in R such that r > x. If there is no r > x, find the first element of the array (wrap around).

Example:
R = ['c', 'f', 'j', 'p', 'v']
if x equals:
'a' => return 'c'
'c' => return 'f'
'k' => return 'p'
'z' => return 'c' (wrap around case)