problem link : CodeChef: Practical coding for everyone
Problem Code: STBMEX
my code 
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
int t,j,i;
cin>>t;
while(t--)
{
long long int n,mex = -1,res = 0,flag = 0;
cin>>n;
long long int a[n];
for(i = 0; i<n;i++)
cin>>a[i];
sort(a,a+n);
long num = 0;
for(i = 0; i<n - 1;i++)
{
if(a[i] == a[i+1])
continue;
if(a[i] == num)
num++;
else
{
flag = 1;
mex = num;
break;
}
}
if(flag == 0)
{
if(a[i] == num)
mex = num + 1;
else
mex = num;
}
// cout<<mex<<endl;
long long int temp = mex - 1;
j= 1;
if(mex == 0)
cout<<a[0] - 1<<endl;
else if(mex == 1)
cout<<-1<<endl;
else
{
for( i = mex;i<n - 1;i++)
{
if(a[i] == a[i+1])
continue;
if(a[i] == a[i + 1] - 1)
{
//cout<<a[i]<<" "<<j<<endl;
j++;
if(j == temp)
res++;
}
else
j = 1;
}
cout<<res<<endl;
}
}
return 0;
}
,