说明：  使用f[i][j]代表从i到j所花费最小费用，则将从i到j路径变成i->z->j，k代表要走的站点数量，起始位置为i所以终点位置为：i+k，保证最后一段为k所以循环截止到i<=n-k，z就是中间站点。如果这样比f[i][j]小，则交换值
(Using f[i][j] to represent the minimum cost from I to j will turn I from J to i->z->j, K represents the number of sites to go, and the initial location is I, so the end point is i+k, which ensures the last segment is k, so the cycle ends to i<=n-k, and the middle point is the middle. If this is smaller than f[i][j], exchange values)