输入3 2 1，运行结果

2721

#include <stdio.h>

void sub(int n,int uu[])
{
    int t;
    t = uu[n--];
    t+=3*uu[n];
    n=n++;
    if(t>=10)
    {
        uu[n++] = t/10;
        uu[n] = t%10;
    }else
        uu[n] = t;
}

void main()
{
    int i,n,aa[10] ={0,0,0,0,0,0};
    scanf("%d%d%d",&n,&aa[0],&aa[1]);  //n=3    aa={2,1,0,0,0,0}
    for(i=1;i<n;i++)
        sub(i,aa);
    for(i=0;i<=n;i++)
        printf("%d",aa[i]);
    printf("\n");
}

2721