What is wrong with my code ?
Am i using a wrong algo ?

/*1818.cpp*/
#include<iostream>
#include<string>
#include<string.h>
#define MAXN 2000
#define MAXNCHAR 4
using namespace std;

string sum(const string &a,const string &b)
{
    string res="";
    unsigned carry=0,
        nA=a.size(),
        nB=b.size();
    while(nA>0 && nB>0)
    {
        res.insert(res.begin(),(a[nA-1]-'0'+b[nB-1]-'0'+carry)%10+'0');
        carry=(a[nA-1]-'0'+b[nB-1]-'0'+carry)/10;
        nA--;
        nB--;
    }
    while(nA>0)
    {
        res.insert(res.begin(),(a[nA-1]-'0'+carry)%10+'0');
        carry=(a[nA-1]-'0'+carry)/10;
        nA--;
    }
    while(nB>0)
    {
        res.insert(res.begin(),(b[nB-1]-'0'+carry)%10+'0');
        carry=(b[nB-1]-'0'+carry)/10;
        nB--;
    }
    if(carry>0)
        res.insert(res.begin(),carry+'0');
    return res;
}

string mult1(const string &a,char b)
{
    string res="";
    unsigned i,nA=a.size(),
        carry=0;
    if(b=='0')
    {
        res="0";
        return res;
    }
    for(i=0;i<nA;i++)
    {
        res.insert(res.begin(),((a[nA-1-i]-'0')*(b-'0')+carry)%10+'0');
        carry=((a[nA-1-i]-'0')*(b-'0')+carry)/10;
    }
    while(carry>0)
    {
        res.insert(res.begin(),carry%10+'0');
        carry/=10;
    }
    return res;
}

string mult(const string &a,const string &b)
{
    unsigned i,j,nB=b.size();
    string tmp,res="0";
    for(i=0;i<nB;i++)
    {
        tmp=mult1(a,b[nB-1-i]);
        for(j=0;j<i;j++)
            tmp.insert(tmp.end(),'0');
        res=sum(res,tmp);
    }
    return res;
}

string x,arr[MAXN];
char l=1;
unsigned n;

void read()
{
    unsigned i;
    char nChar[MAXNCHAR+1];
    cin>>n;
    for(i=0;i<n;i++)
    {
        cin>>arr[i];
        l=l && arr[i]=="0";
    }
    sprintf(nChar,"%u",n);
    x.assign(nChar);
}

string solve()
{
    string res;
    unsigned i;
    if(l==0)
    {
        res=arr[n-1];
        for(i=1;i<n;i++)
        {
            res=mult(res,x);
            res=sum(res,arr[n-1-i]);
        }
        return res;
    }
    res=x;
    for(i=1;i<n;i++)
        res=mult(res,x);
    return res;
}

void write()
{
    cout<<solve()<<endl;
}

int main()
{
    read();
    write();
    return 0;
}

Edited by author 08.02.2011 01:51

Edited by author 08.02.2011 01:52

Edited by author 10.02.2011 16:32

the output for
3
0 0 0
is 9 or 0 ? I've got WA on #3

and for
3
0 0 1
is 9 too?

What the output for
2000

1999 of '0'
and one '1'