IMP find first missing number

#include <iostream>
using namespace std;

int main() {
    int i,j,t,n,x;
    cin>>t;
    while(t--)
    {
        cin>>n;
        int a[n];
        for(i=0;i<n;i++)cin>>a[i];
        if(n==0)cout<<1<<endl;
        else{
            int cnt=0;
            for(i=0;i<n;i++)
            {
                if(a[i]==1)cnt++;
                if(a[i]<=0||a[i]>n)a[i]=1;
            }
            if(cnt==0)cout<<1<<endl;
            else{
                for(i=0;i<n;i++)
                {
                    j=abs(a[i])-1;
                    if(a[j]>0)a[j]=-1*a[j];
                }

                for(i=0;i<n;i++)
                {
                    if(a[i]>0)break;
                }

                cout<<i+1<<endl;
            }
        }
    }
    return 0;
}