A7877.阶乘之和 题解

经典高精度

#include <bits/stdc++.h>
using namespace std;
int n,a[2200],b[2200],c[2200];
void fact(int n,int c[]){
	c[0]=1;
	for (int i=1;i<=n;i++){
		for (int j=0;j<=2100;j++){
			c[j]*=i;
		}
		for (int j=0;j<=2100;j++){
			c[j+1]+=c[j]/10;
			c[j]%=10;
		}
	}
}
void add(int x[],int y[],int z[]){
	for (int i=0;i<2100;i++){
		int t=x[i]+y[i]+z[i];
		z[i+1]=t/10;
		z[i]=t%10;
	}
}
int main(){
	cin>>n;
	for (int i=1;i<=n;i++){
		fact(i,a);
		add(a,c,b);
		for (int i=0;i<=2100;i++) c[i]=b[i];
		memset(a,0,sizeof(a));
		memset(b,0,sizeof(b));
	}
	int x=2100;
	while (x>0&&c[x]==0) x--;
	for (int i=x;i>=0;i--) cout<<c[i];
	return 0;
}