Friendlist in AIO challenge

I am working on the problem at http://ift.tt/2hvckXG. Below is the code which I came up. Sample data executed successfully but when I submit the code I am able to execute only 71% of the test cases. Not sure where went wrong. Please guide me.

#include <bits/stdc++.h>

using namespace std;

int main(){
    freopen("listin.txt","r",stdin);
    freopen("listout.txt","w",stdout);
    int f=0, a= 0, b= 0, m = 1;
    long size = 1001;

    cin >> f;
    int arr[size] = {};
    int x[size] = {};
    for(int i = 0; i < f; i++){
        cin >> a >> b;
        arr[a] += 1;
        arr[b] += 1;
    }
    for(int j = 0; j < size; j++){
        if(m < arr[j]){
            m = arr[j];
        }
    }
    for(int j = 0; j < size; j++){
        if(m == arr[j]){
            x[j] = j;
        }
    }
    for(int k = 0; k < size; k++){
        if(x[k] > 0)
            cout << x[k] << endl;
    }

    return 0;
}