Runtime Error encountered while running on Online Judges

I have been solving this problem on SPOJ . One of the most frequent problems encountered while using online judges is Runtime Error. You never know which case leads to a Segmentation fault. Please help me figure out why does the code below correspond to a Runtime Error or Segmentation Fault even though I have ensured that every possible case runs properly on my linux gcc.

#include <iostream>
#include <math.h>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <string>
#include <vector>


using namespace std;

int main ()
{
int t,i;
cin >> t;
string s;
vector<int> A;


while(t--)
{   

    cin >> s;
    int n=s.size();
    int temp;

    if (n!=1) 
    {
        for(i=0;i<s.size();i++)
        {
            A.push_back(s.at(i)-'0');
        }

        if(n%2!=0)
        {   i = (n-1)/2;

            while(A[i-1]==A[n-i])
                i--;
            i--;
        }
        else
        {   i=n/2-1;

            while(A[i]==A[n-1-i])
                i-- ; 

        }


        if (A[i]<A[n-i-1])
        {   if ((n%2)!=0)
                {A[n/2] = A[n/2] + 1;
                A[n-i-1] = A[i];}
            else
                {
                    A[n/2-1] = A[n/2-1]+1;
                    A[n/2] = A[n/2-1];
                    A[n-i-1] = A[i]; 
                }
        }
        else
            A[n-i-1] = A[i];

        while(i--)
            A[n-i-1] = A[i];



        while(!(A.empty()))
        {
            printf("%d",A.back());
            A.pop_back();
        }
    }
    else
        cout << s;

}

}