Given a list of sorted characters letters containing only lowercase letters, and given a target letter target, find the smallest element in the list that is larger than the given target.
Letters also wrap around. For example, if the target is target = 'z' and letters = ['a', 'b'], the answer is 'a'.
1 因为是sorted的，所以只要依次遍历，找到第一个比target大的数，就是所求
2 如果找不到，返回第一个值
