Write a program to reverse an array or string

08 Sep, 2020
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
  
Another example to reverse a string: 
Below is the implementation of the above approach :  
C++
C
Java
Python
C#
PHP
// Iterative C++ program to reverse an array#include <bits/stdc++.h>using namespace std; /* Function to reverse arr[] from start to end*/void rvereseArray(int arr[], int start, int end){    while (start < end)    {        int temp = arr[start];        arr[start] = arr[end];        arr[end] = temp;        start++;        end--;    }}     /* Utility function to print an array */void printArray(int arr[], int size){   for (int i = 0; i < size; i++)   cout << arr[i] << " ";    cout << endl;} /* Driver function to test above functions */int main(){    int arr[] = {1, 2, 3, 4, 5, 6};         int n = sizeof(arr) / sizeof(arr[0]);     // To print original array    printArray(arr, n);         // Function calling    rvereseArray(arr, 0, n-1);         cout << "Reversed array is" << endl;         // To print the Reversed array    printArray(arr, n);         return 0;}
// Iterative C program to reverse an array#include<stdio.h> /* Function to reverse arr[] from start to end*/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 */void printArray(int arr[], int size){  int i;  for (i=0; i < size; i++)    printf("%d ", arr[i]);   printf("\n");} /* Driver function to test above functions */int main(){    int arr[] = {1, 2, 3, 4, 5, 6};    int n = sizeof(arr) / sizeof(arr[0]);    printArray(arr, n);    rvereseArray(arr, 0, n-1);    printf("Reversed array is \n");    printArray(arr, n);       return 0;}
// Iterative java program to reverse an// arraypublic 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);            }} // This code is contributed by Sam007
# Iterative python program to reverse an array # Function to reverse A[] from start to enddef reverseList(A, start, end):    while start < end:        A[start], A[end] = A[end], A[start]        start += 1        end -= 1 # Driver function to test above functionA = [1, 2, 3, 4, 5, 6]print(A)reverseList(A, 0, 5)print("Reversed list is")print(A)# This program is contributed by Pratik Chhajer
// Iterative C# program to reverse an// arrayusing System; 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++)            Console.Write(arr[i] + " ");                 Console.WriteLine();    }         // Driver function    public static void Main()    {        int []arr = {1, 2, 3, 4, 5, 6};        printArray(arr, 6);        rvereseArray(arr, 0, 5);        Console.Write("Reversed array is \n");        printArray(arr, 6);    }} // This code is contributed by Sam007
<?php// Iterative PHP program// to reverse an array /* Function to reverse$arr from start to end*/function rvereseArray(&$arr, $start,                       $end){    while ($start < $end)    {        $temp = $arr[$start];        $arr[$start] = $arr[$end];        $arr[$end] = $temp;        $start++;        $end--;    }}     /* Utility function to   print an array */function printArray(&$arr, $size){for ($i = 0; $i < $size; $i++)echo $arr[$i] . " "; echo "\n";} // Driver code$arr = array(1, 2, 3, 4, 5, 6); // To print original arrayprintArray($arr, 6); // Function callingrvereseArray($arr, 0, 5); echo "Reversed array is" ."\n"; // To print the Reversed arrayprintArray($arr, 6); // This code is contributed// by ChitraNayal?>
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 :  
C++
C
Java
Python
C#
PHP
// Recursive C++ program to reverse an array#include <bits/stdc++.h>using namespace std; /* Function to reverse arr[] from start to end*/void rvereseArray(int arr[], int start, int end){    if (start >= end)    return;         int temp = arr[start];    arr[start] = arr[end];    arr[end] = temp;         // Recursive Function calling    rvereseArray(arr, start + 1, end - 1);}      /* Utility function to print an array */void printArray(int arr[], int size){   for (int i = 0; i < size; i++)   cout << arr[i] << " ";    cout << endl;} /* Driver function to test above functions */int main(){    int arr[] = {1, 2, 3, 4, 5, 6};         // To print original array    printArray(arr, 6);         // Function calling    rvereseArray(arr, 0, 5);         cout << "Reversed array is" << endl;         // To print the Reversed array    printArray(arr, 6);         return 0;}
// Recursive C program to reverse an array#include <stdio.h> /* Function to reverse arr[] from start to end*/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 */void printArray(int arr[], int size){  int i;  for (i=0; i < size; i++)    printf("%d ", arr[i]);   printf("\n");} /* Driver function to test above functions */int main(){    int arr[] = {1, 2, 3, 4, 5, 6};    printArray(arr, 6);    rvereseArray(arr, 0, 5);    printf("Reversed array is \n");    printArray(arr, 6);       return 0;}
// Recursive Java Program to reverse an arrayimport 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);    }}/*This article is contributed by Devesh Agrawal*/
# Recursive python program to reverse an array # Function to reverse A[] from start to enddef reverseList(A, start, end):    if start >= end:        return    A[start], A[end] = A[end], A[start]    reverseList(A, start+1, end-1) # Driver function to test above functionA = [1, 2, 3, 4, 5, 6]print(A)reverseList(A, 0, 5)print("Reversed list is")print(A)# This program is contributed by Pratik Chhajer
// C# program to reverse an arrayusing System; class GFG{    /* 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++)            Console.Write(arr[i] + " ");             Console.WriteLine("");    }     // Driver Code    public static void Main ()    {        int []arr = {1, 2, 3, 4, 5, 6};                 printArray(arr, 6);        rvereseArray(arr, 0, 5);                 Console.WriteLine("Reversed array is ");        printArray(arr, 6);    }} // This code is contributed by Sam007
<?php// Iterative PHP program// to reverse an array /* Function to reverse$arr from start to end*/function rvereseArray(&$arr,                       $start, $end){    if ($start >= $end)    return;         $temp = $arr[$start];    $arr[$start] = $arr[$end];    $arr[$end] = $temp;     //Recursive Function calling    rvereseArray($arr, $start + 1,                       $end - 1);}     /* Utility functionto print an array */function printArray(&$arr, $size){for ($i = 0; $i < $size; $i++)echo $arr[$i] . " "; echo "\n";} // Driver code$arr = array(1, 2, 3, 4, 5, 6); // To print original arrayprintArray($arr, 6); // Function callingrvereseArray($arr, 0, 5); echo "Reversed array is" ."\n"; // To print the Reversed arrayprintArray($arr, 6); // This code is contributed// by ChitraNayal?>
Output : 
1 2 3 4 5 6 
Reversed array is 
6 5 4 3 2 1 



Time Complexity : O(n)
Another Approach: Using Python List slicing
Python3
def reverseList(A):  print( A[::-1])     # Driver function to test above functionA = [1, 2, 3, 4, 5, 6]print(A)print("Reversed list is")reverseList(A) 
Output:
[1, 2, 3, 4, 5, 6]
Reversed list is
[6, 5, 4, 3, 2, 1]
 
Please write comments if you find any bug in the above programs or other ways to solve the same problem. 
Sam007
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