Codeforces Round #Pi (Div. 2) C. Geometric Progression | 书脊



Codeforces Round #Pi (Div. 2) C. Geometric Progression | 书脊

Codeforces Round #Pi (Div. 2) C. Geometric Progression

原题: http://codeforces.com/contest/567/problem/C

题目大意: 给一个n大小数组, 问其中的子序列(非连续)能组成多少个已p为公比的等比序列, 且长度为3.

分析: 这题卡了好久, 我用找等差数列的方法改的. 后来总是过不了test. 可是怎么看我的是O(nlgn)的复杂度, 所以过不了大的test case 肯定是有O(n)的. 于是开始想啊想啊….最后想到了是用等比中项的性质.


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