poj 2531(按位暴力枚举2^20很easy)

Network Saboteur

Time Limit: 2000MS Memory Limit: 65536KTotal Submissions: 8492 Accepted: 3928

Description

A university network is composed of N computers. System administrators gathered information on the traffic between nodes, and carefully divided the network into two subnetworks in order to minimize traffic between parts.
A disgruntled computer science student Vasya, after being expelled from the university, decided to have his revenge. He hacked into the university network and decided to reassign computers to maximize the traffic between two subnetworks.
Unfortunately, he found that calculating such worst subdivision is one of those problems he, being a student, failed to solve. So he asks you, a more successful CS student, to help him.
The traffic data are given in the form of matrix C, where Cij is the amount of data sent between ith and jth nodes (Cij = Cji, Cii = 0). The goal is to divide the network nodes into the two disjointed subsets A and B so as to maximize the sum ∑Cij (i∈A,j∈B).

Input

The first line of input contains a number of nodes N (2 <= N <= 20). The following N lines, containing N space-separated integers each, represent the traffic matrix C (0 <= Cij <= 10000).
Output file must contain a single integer -- the maximum traffic between the subnetworks.

Output

Output must contain a single integer -- the maximum traffic between the subnetworks.

Sample Input

30 50 3050 0 4030 40 0

Sample Output

90

Source

Northeastern Europe 2002, Far-Eastern Subregion'

刚开始考虑用搜索之类的办法，结果发现n<=20，直接2^20枚举就好了。
贡献了一次WA，需要注意的是 移位运算符<<运算优先级是低于减法运算符- 的。
因此 1<<n - 1 和 1<<(n-1)是等价的。
其实只需要枚举[1, 1<<n-1] 就可以。
因为后面的跟前面的是对称的。
例如：
001   110
010   101
011   100