
39  ZOJ Monthly, September 2005  1007
An arithmetic series consists of a sequence of terms such that each term minus its immediate predecessor gives the same result. For example, the sequence 3, 7, 11, 15 is the terms of the arithmetic series 3+7+11+15; each term minus its predecessor equals 4. (Of course there is no requirement on the first term since it has no predecessor.) Given a collection of integers, we want to find the longest arithmetic series that can be formed by choosing a subcollection (possibly the entire collection). Input There are multiple cases, and each case contains 2 lines: the first line contains the count of integers (between 2 and 1000 inclusive), the following line contains all the integers (between 1,000,000,000 and 1,000,000,000 inclusive) separated by one or more spaces. Output Print a single number for each case in a single line. Sample Input 7 3 8 4 5 6 2 2 4 1 5 1 3 4 10 20 10 10 Sample Output 5 3 3 Author: JIN, Tianpeng 