Little Boxes hdu - 6225

#include <stdio.h>
#include <stdlib.h>
#include <string.h> 
#define N 1001
char a[N],b[N],c[N],d[N];
int a1[N],b1[N],c1[N],d1[N];
int main()  {
    int n;
    scanf("%d",&n);
    while (n--) {
        scanf("%s",a);
        scanf("%s",b);
        scanf("%s",c);
        scanf("%s",d);
        int k=0,t;
        for (int i=N-1;i>=0;i--)
            if (a[i]!='\0')
                a1[k++]=a[i]-'0';
        k=0;
        for (int i=N-1;i>=0;i--)
            if (b[i]!='\0')
                b1[k++]=b[i]-'0';
        k=0;
        for (int i=N-1;i>=0;i--)
            if (c[i]!='\0')
                c1[k++]=c[i]-'0';
        k=0;
        for (int i=N-1;i>=0;i--)
            if (d[i]!='\0')
                d1[k++]=d[i]-'0';
        for (int i=0;i<N;i++)   {
            a1[i]=a1[i]+b1[i];
            if (a1[i]>9)    {
                a1[i+1]++;
                a1[i]-=10;
            }
        }
        for (int i=0;i<N;i++)   {
            c1[i]=c1[i]+d1[i];
            if (c1[i]>9)    {
                c1[i+1]++;
                c1[i]-=10;
            }
        }
        for (int i=0;i<N;i++)   {
            a1[i]=a1[i]+c1[i];
            if (a1[i]>9)    {
                a1[i+1]++;
                a1[i]-=10;
            }
        }
        for (t=N-1;t>=0;t--)
            if (a1[t]!=0)   break;
        if (t==-1)  printf("0");
        else
            for (int i=t;i>=0;i--)
                printf("%d",a1[i]);
        printf("\n");
        memset(a,'\0',sizeof(a));
        memset(b,'\0',sizeof(b));
        memset(c,'\0',sizeof(c));
        memset(d,'\0',sizeof(d));
        memset(a1,0,sizeof(a1));
        memset(b1,0,sizeof(b1));
        memset(c1,0,sizeof(c1));
        memset(d1,0,sizeof(d1));
    }
    return 0;
}