Java 8 Coding challenge: Swapping Characters

Your are given a string of length N. You have to follow some rounds of swapping in the string until the below explained condition is reached.

In each round the, you have to swap the ith and (i+1)th character , then (i+2)th and (i+3)th character and continue till the end of string. In each round , a character from the left will be locked i.e not available for swapping after this round , thus after some rounds of swapping , all the characters will be locked at their final position to stop any more rounds of swapping .

See the sample explanation for more clarity.

Input

The first line of input contains a T, the number of test case. The first line of each Test case will contain an Integer N , denoting the length of the string. The second line of each Test case will contain a string of length N.

Output

For each and every test case, output the string after all positions are locked.

Constraints

1. 0<T<100
2. 0<N<10000

SAMPLE INPUT

2
6
abcdef
3
abc

SAMPLE OUTPUT

bdfeca
bca

Explanation

For the first test case , the given string is "abcdef" and all the characters are unlocked.

1. After First round of swap , the string becomes "badcfe" . Now the leftmost unlocked character is locked i.e the string is now "b_adcfe".
2. After Second round of swap , the string becomes "b_dafce". Now the leftmost unlocked character is locked i.e string is now "bd_afce".
3. After Third round of swap , the string becomes "bdfaec" . Now the leftmost unlocked character is locked i.e the string is now "bdf_aec".
4. After Fourth round of swap , the string becomes "bdfeac" . Now the leftmost unlocked character is locked i.e the string is now "bdfe_ac"..
5. After Fifth round of swap , the string becomes "bdfeca" . Now the leftmost unlocked character is locked i.e the string is now "bdfec_a".
6. In the sixth round no more swap occur as there is only single unlocked character is left.

** ('-' is the partition , where all the characters to its left are locked characters and all the characters to it right are unlocked characters).**

Therefore , we print the string "bdfeca".

Code:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
class Main {
 
 public static void main(String args[] ) throws IOException {
 
 BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
 for(int k=Integer.parseInt(br.readLine());k>0;k--){
 br.readLine();
 String str=br.readLine();
 StringBuilder s=new StringBuilder(str);
 int start=0;
 int end=str.length()-1;
 for(int i=1;i<str.length();i+=2){
 s.setCharAt(start++, str.charAt(i));
 }
 for(int i=0;i<str.length();i+=2){
 s.setCharAt(end--, str.charAt(i));
 }
 System.out.println(s);
 }
 }
 public static void swap(StringBuilder s,int i,int j){
 char c=s.charAt(i);
 char d=s.charAt(j);
 s.setCharAt(i, d);
 s.setCharAt(j, c);
 }
 
}