文档介绍：1789 Truck History Time Limit: 2000MS Memory Limit: 65536K Total Submissions: 12194 Accepted: 4603 Description Advanced Cargo Movement, Ltd. uses trucks of different types. Some trucks are used for vegetable delivery, other for furniture, or for bricks. pany has its own code describing each type ofa truck. The code is simply a string of exactly seven lowercase letters (each letter on each position has a very special meaning but that is unimportant for this task). At the beginning pany's history, just a single truck type was used but later other types were derived from it, then from the new types another types were derived, and soon. Today, ACM is rich enough to pay historians to study its history. One thing historians tried to find out is so called derivation plan -- . how the truck types were derived. They defined the distance of truck types as the number of positions with different letters in truck type codes. They also assumed that each truck type was derived from exactly one other truck type (except for the first truck type which was not derived from any other type). The quality ofa derivation plan was then defined as1/Σ(to,td) d(t o,t d) where the sum goes over all pairs of types in the derivation plan such that t o is the original type and t d the type derived from it and d(t o,t d) is the distance of the types. Since historians failed, you are to write a program to he