OJ平台中的一个数组初始化问题

Today in KBW, N mice from different cities are standing in a line. Each city is represented by a lowercase letter. The distance between adjacent mice (e.g. the 1^st and the 2^nd mouse, the N−1^th and the N^th mouse, etc) are exactly 1. Two mice are friends if they come from the same city.

The closest friends are a pair of friends with the minimum distance. Help us find that distance.