Java 8 solution

public static int sockMerchant(int n, List ar) {

     Set<Integer> sock = new HashSet<>();
     int pairs = 0;

     for(int ele: ar){
         if(sock.contains(ele)){
             pairs++;
             sock.remove(ele);             
         } else{
            sock.add(ele);   
         }
     }
     return pairs;
}