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 因為是sorted的竿裂，所以只要依次遍歷许蓖，找到第一個比target大的數(shù)吱窝，就是所求
2 如果找不到惑艇，返回第一個值
