http://codeforces.com/problemset/problem/630/I
简单的排列组合,推式子技巧:举一个小样例,看着推,别抽象着推,容易错
#include <iostream> #include <cstdio> #include <queue> #include <algorithm> #include <cmath> #include <cstring> #define inf 2147483647 #define N 1000010 #define p(a) putchar(a) #define For(i,a,b) for(long long i=a;i<=b;++i) using namespace std; long long n; void in(long long &x){ long long y=1;char c=getchar();x=0; while(c<'0'||c>'9'){if(c=='-')y=-1;c=getchar();} while(c<='9'&&c>='0'){ x=(x<<1)+(x<<3)+c-'0';c=getchar();} x*=y; } void o(long long x){ if(x<0){p('-');x=-x;} if(x>9)o(x/10); p(x%10+'0'); } long long ksm(long long a,long long b){ long long r=1; while(b>0){ if(b&1) r=r*a; a=a*a; b>>=1; } return r; } int main(){ in(n); o((9*n-3)*ksm(4,n-3)); return 0; }