HDU4602+推导公式
手动列出前5项 可发现规律
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 /* 2 推导公式 3 a[n] = 2^(n-1) + (n-2)*2^(n-3) 4 */ 5 #include<stdio.h> 6 #include<math.h> 7 #include<algorithm> 8 using namespace std; 9 typedef long long int64; 10 const int64 mod = 1e9+7; 11 12 int64 FastPow( int64 n,int64 m ){//n^m 13 int64 sum = 1; 14 while( m>=1 ){ 15 if( m%2==1 ){ 16 sum *= n; 17 sum %= mod; 18 } 19 n *= n; 20 n %= mod; 21 m/=2; 22 } 23 return sum; 24 } 25 26 int main(){ 27 int T; 28 scanf("%d",&T); 29 while( T-- ){ 30 int64 n,k; 31 //scanf("%lld%lld",&n,&k); 32 scanf("%I64d%I64d",&n,&k); 33 if( k>n ){ 34 printf("0\n"); 35 continue; 36 } 37 if( k==n ){ 38 printf("1\n"); 39 continue; 40 } 41 int64 delta = k-1; 42 int64 Index = n-delta; 43 44 if( Index==1 ){ 45 printf("1\n"); 46 continue; 47 } 48 if( Index==2 ){ 49 printf("2\n"); 50 continue; 51 } 52 53 //printf("%lld\n",(FastPow2(2,Index-1)%mod+FastPow2(2,Index-3)*(Index-2)%mod)%mod); 54 printf("%I64d\n",(FastPow(2,Index-1)%mod+FastPow(2,Index-3)*(Index-2)%mod)%mod); 55 } 56 return 0; 57 }
keep moving...