HDU 5363 Key Set (快速幂）

Key Set

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 131072/131072 K (Java/Others)
Total Submission(s): 1709    Accepted Submission(s): 905

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 ofS are key set.

 

Input

There are multiple test cases. The first line of input contains an integerT(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.

 

Output

For each test case, output the number of key sets modulo 1000000007.

 

Sample Input

41234

 

Sample Output

0137

 

Author

zimpha@zju

 

Source

2015 Multi-University Training Contest 6

 

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题意：一个集合元素为1到n,求这个集合有多少个非空子集，并且子集的元素之和为偶数

思路：2 ^ ( n - 1) -1;

纠结为什么我用( 2 ^ n ) / 2 -1.就不对.

#include<stdio.h>__int64 mod(__int64 a,__int64 b,__int64 c){__int64 res=1%c;__int64 t=a%c;while(b){if(b%2){res=res*t%c;}t=t*t%c;    b=b/2;}return res;}int main(){__int64 t,n;scanf("%I64d",&t);while(t--){scanf("%I64d",&n);__int64 ans=mod(2,n-1,1000000007);printf("%I64d\n",ans-1);}return 0;}