Rabin and Karp proposed a string-matching algorithm that performs well in prac- tice and that also generalizes to other algorithms for related problems,such as two-dimensional pattern matching.The Rabin-Karp algorithm uses (m)prepro- cessing time,and its worst-case running time is((n-m+1)m).Based on certain assumptions,however,its average-case running time is better. 问题10： 这是书上开始介绍Rabin-Karp 算法时的第一段话，现在你能 解释一下了吗？