2015多校联合第六场hdu5363Key Set 快速幂+yy
Problem Description
soda has a set S with n integers {1,2,…,n}. A set is called key set if the sum of integers in the set is an even number. He wants to know how many nonempty subsets of S are key set.
Input
There are multiple test cases. The first line of input contains an integer T (1≤T≤105), indicating the number of test cases. For each test case:
The first line contains an integer n (1≤n≤109), the number of integers in the set.
The first line contains an integer n (1≤n≤109), the number of integers in the set.
Output
For each test case, output the number of key sets modulo 1000000007.
Sample Input
4 1 2 3 4
Sample Output
0 1 3 7
纯数学题 ，尝试着找公式，失败，据说这是这是找规律的题，貌似之前也见到过类似的写几个数就看出来端倪的orz
这个题更是……
#include <iostream>
#include<cstdio>
#include<cstring>
using namespace std;
#define MOD 1000000007
long long mod(long long a,long long b)
{
long long ans=1;
while(b)
{
if(b&1) ans=(ans*a)%MOD;
b/=2;
a=(a*a)%MOD;
}
return ans;
}
int main()
{
int t;
long long n;
cin>>t;
while(t--)
{
cin>>n;
cout<<mod(2,n-1)-1<<endl;
}
return 0;
}
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