全WA了，哪位大佬help一下我

#include<bits/stdc++.h>
using namespace std;
int a[40005],sum[40005];
int main(){
   int n,len=1,lensum=1;
   cin>>n;
   a[0]=1;
   for(int k=0;k<n;k++){
   	for(int i=0;i<len;i++){
   		a[i]*=k;
   	}
   	for(int i=0;i<len;i++){
   		a[i+1]+=a[i]/10;
   		a[i]%=10;
   	}
   	while(a[len]){
   		a[len+1]+=a[len]/10;
   		a[len]%=10;
   		len++;
   	}
   	lensum=max(lensum,len);
   	for(int i=0;i<lensum;i++){
   		sum[i]+=a[i];
   	}
   	for(int i=0;i<lensum;i++){
   		sum[i+1]+=sum[i]/10;
   	}
   	while(sum[lensum]){
   		sum[lensum+1]+=sum[lensum]/10;
   		sum[len]%=10;
   		lensum++;
   	}
   }
   for(int i=lensum-1;i>=0;i--){
   	cout<<sum[i];
   }
   return 0;
}