C Program to Reverse an Array or String

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

// 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("
");
}

/* 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
");
    printArray(arr, n);
    return 0;
}

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

// 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;
}

Output :

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

Time Complexity : O(n)

Please write comments if you find any bug in the above programs or other ways to solve the same problem.