Question

i was trying to do this problem but i could not able to get it accepted , its input is large so i thought to take it with strings but still i am getting wrong answer .
problem statement is here BISHOPS

here is my code:

#include<iostream>
#include<string.h>
#include<cstdio>
#include<vector>

using namespace std;

int main()
{
    string s;
    int z;
    cin>>s;
    z = s.length();
    int i;
    vector<int>v;
    int arr[z];
    for(i=0;i<z;i++)
    {
        arr[i] = (s[i]-'0');
    }

    if ((arr[0] == 0 || arr[0] ==  1) && z == 1)
    {
        printf("%d",arr[0]);
    }
    else
    { 
        int carry = 0;
        for(i=z-1;i>=0;i--)
        {
            int x = (carry + 2*arr[i])%10;
            v.push_back(x);
            carry = ( 2*arr[i] )/10;
        }
        if(carry > 0)
        {
            v.push_back(carry);
        }

        int t = v.size();
        int g=0;
        if(v[g] >=2)
        {
            v[g] = v[g]-2;
        }
        else
        {
            v[g] = 8;
            g++;
            while(v[g] == 0)
            {
                v[g] = 9;
                g++;
            }
            v[g] = v[g] -1;
        } 


        if(v[t-1] == 0)
        {
            for(i=t-2;i>=0;i--)
                {
                    printf("%d",v[i]);
                }
        }
        else
        {
            for(i=t-1;i>=0;i--)
            {
                printf("%d",v[i]);
            }
        }
    }

        return 0;
}

question is to find max non attackable bishops in the problem for nxn chess board ,i think i am doing right because except for 1×1 for every oyher chess board max non attacakable bishops will be 2*n-2.but still spoj is giving wrong answer

Answer 1

You have integer overflow for all inputs larger than INT_MAX:

int y=0,i,z;
z = s.length();
for(i=0;i<z;i++)
{
    y = y*10 + (s[i]-'0');
}

You are trying to convert the input to an int, which typically is a signed 32-bit type with INT_MAX equal to 2147483647 = 2³¹-1. Whenever the input is larger than that, at the tenth digit you have overflow (which, by the way, is undefined behaviour).

You need a way to represent/handle numbers larger than that, up to 10¹⁰⁰. No standard integer type in C or C++ is wide enough to handle that. You have to roll your own big-integer type (since SPOJ presumably doesn’t link the code with GMP). For this problem, something very simple works well enough.