ABC-String

Dear all , I am getting WA for the below solution .While local testing , I am getting correct Answer .

Any pointer will help me to understand the issue in below implementation

Thanks rajesh

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#include<iostream>
#include<vector>
#include<algorithm>
#include<string.h>
#include<stdio.h>
#define STRING_LENGTH 1000005
char data[STRING_LENGTH];
using namespace std;
vector<pair<long,long> >v(STRING_LENGTH);
int main()
{
  freopen("input.txt","r",stdin);
   memset(data,'\0',STRING_LENGTH);
   cin >> data;
    long d1 = 0;
    long d2 = 0;
   unsigned long i = 0;
    v[0].first = 0;
    v[0].second = 0;
   while(data[i] != '\0')
   {
       if (data[i] == 'A')
       {
           d1++;
       }
       else if (data[i] == 'B')
       {
           d2++;
       }
       else if ( data[i] == 'C')
       {
           d1--;
           d2--;
       }
       v[i+1] = make_pair(d1,d2);
       i++;
   }
   sort(v.begin(),v.begin()+i+1);
   unsigned long index = i;
    long a = -1;
    long b = -1;
   unsigned long counter = 0;
   unsigned long sum = 0;
   i = 0;
   while(i <= index )
   {
       if (v[i].first == a && v[i].second == b)
       {
           counter++;
       }
       else
       {
           sum += ((counter)*(counter-1))/2;
           counter = 1;
           a = v[i].first;
           b = v[i].second;
       }

       i++;
   }
   sum += ((counter)*(counter-1))/2;
   cout<<sum<<endl;
   v.clear();
   sum = 0;

   return 0;
}


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