#include<stdio.h>
#include<deque>
#include<algorithm>
using namespace std;
class node{
    public:
    int x,y,num;
}dum;
bool func(node i,node j)
{
    if(i.y > j.y)
    return true;
    return false;
}
int main()
{
    deque<node> city;
    int n;    scanf("%d",&n);
    for(int i=0;i<n;i++)
    {
        scanf("%d %d",&dum.x,&dum.y);
        dum.num = (i+1);
        city.push_back(dum);
    }
    if(n == 0)
    {printf("0");return 0;}
    sort(city.begin(),city.end(),func);
    for(int i=0;i<n;i+=2)
    {
        printf("%d %d\n",city[i].num,city[i+1].num);
    }
}

your code is wrong http://ideone.com/2mv3As

it prints
3 4
1 2

instead of
1 3
2 4

for the test from description of the problem

perhaps it gaves AC so your code is right, tests are wrong or problem description is not completed