EXTRAN.java

package com.company;

import java.util.Arrays;
import java.util.Scanner;

import static java.util.Arrays.binarySearch;

public class EXTRAN {



    public static void main(String[] args) {
	// write your code here
        // your code goes here
        Scanner sc = new Scanner(System.in);
        int t = sc.nextInt();
        while(t-->0){
            int n = sc.nextInt();
            int[] arr = new int[n];

            for(int i=0;i<n;i++)
                arr[i]=sc.nextInt();

            System.out.println(getFaultNum(arr,n));

        }
    }

    private static int getFaultNum(int[] arr, int n) {

        Arrays.sort(arr);

        //check for duplicates
        for(int i=1;i<n;i++)
            if(arr[i-1]==arr[i])
                return arr[i];
        //binarySearch based solution
        int key = arr[0];
        for(int i=1;i<n;i++){
            if(binarySearch(arr,++key)>0)
            {
                key=arr[i];
                continue;
            }
            else if(i==n-1)
                return arr[n-1];
            else
                return arr[i-1];
        }

        return -1;
    }


}

Hmmm... I can't understand: for what this code?