Reverse an array or string

Software Development
#1

Given an array (or string), the task is to reverse the array/string.
Examples :

Input : arr[] = {1, 2, 3}
Output : arr[] = {3, 2, 1}

Input : arr[] = {4, 5, 1, 2}
Output : arr[] = {2, 1, 5, 4}

Iterative way :

  1. Initialize start and end indexes as start = 0, end = n-1
  2. In a loop, swap arr[start] with arr[end] and change start and end as follows :
    start = start +1, end = end – 1

Below is the implementation of the above approach :

// Iterative java program to reverse an

// array

public class GFG {

/* Function to reverse arr[] from

start to end*/

static void rvereseArray( int arr[],

int start, int end)

{

int temp;

while (start < end)

{

temp = arr[start];

arr[start] = arr[end];

arr[end] = temp;

start++;

end--;

}

}

/* Utility that prints out an

array on a line */

static void printArray( int arr[],

int size)

{

for ( int i = 0 ; i < size; i++)

System.out.print(arr[i] + " " );

System.out.println();

}

// Driver code

public static void main(String args[]) {

int arr[] = { 1 , 2 , 3 , 4 , 5 , 6 };

printArray(arr, 6 );

rvereseArray(arr, 0 , 5 );

System.out.print( "Reversed array is \n" );

printArray(arr, 6 );

}

}

Output :

1 2 3 4 5 6 Reversed array is 6 5 4 3 2 1

Time Complexity : O(n)
Recursive Way :

  1. Initialize start and end indexes as start = 0, end = n-1
  2. Swap arr[start] with arr[end]
  3. Recursively call reverse for rest of the array.

Below is the implementation of the above approach :

// Recursive Java Program to reverse an array

import java.io.*;

class ReverseArray {

/* Function to reverse arr[] from start to end*/

static void rvereseArray( int arr[], int start, int end)

{

int temp;

if (start >= end)

return ;

temp = arr[start];

arr[start] = arr[end];

arr[end] = temp;

rvereseArray(arr, start+ 1 , end- 1 );

}

/* Utility that prints out an array on a line */

static void printArray( int arr[], int size)

{

for ( int i= 0 ; i < size; i++)

System.out.print(arr[i] + " " );

System.out.println( "" );

}

/*Driver function to check for above functions*/

public static void main (String[] args) {

int arr[] = { 1 , 2 , 3 , 4 , 5 , 6 };

printArray(arr, 6 );

rvereseArray(arr, 0 , 5 );

System.out.println( "Reversed array is " );

printArray(arr, 6 );

}

}

Output :

1 2 3 4 5 6 Reversed array is 6 5 4 3 2 1

Time Complexity : O(n)