How to Reverse a String in C?

In C, a string is a sequence of characters terminated by a null character (\0). Reversing a string means changing the order of the characters such that the characters at the end of the string come at the start and vice versa. In this article, we will learn how to reverse a string in C.

Example:

Input:
char myStr[] = "Hello";

Output:
Reversed String: olleH

Reverse a String in C

To reverse a string in C, we can use a two-pointer approach in which we will use two pointers pointing to the first and last index. In every iteration, swap the characters at these positions and move the pointers toward each other until they meet or cross to get the reversed string.

Approach:

1. Declare two pointers, start(pointing to the beginning) and end (pointing to the end).
2. Swap the characters at the positions pointed by start and end.
3. Increment start and decrement end.
4. Repeat steps 2 and 3 until start is greater than or equal to end.

C Program to Reverse a String

The below program demonstrates how we can reverse a string in C.

// C program to reverse a string

#include <stdio.h>
#include <string.h>

int main()
{

    // Initialize a string
    char str[100] = "Hello, World!";

    // pointing to the start of string
    char* start = str;
    // pointing to the end of string
    char* end = str + strlen(str) - 1;

    // Print the original string
    printf("Original string: %s\n", str);

    // Reverse the string
    while (start < end) {

        // perform swap
        char temp = *start;
        *start = *end;
        *end = temp;
        // increment start
        start++;
        // decrement end
        end--;
    }

    // Print the reversed string
    printf("Reversed string: %s\n", str);

    return 0;
}

Original string: Hello, World!
Reversed string: !dlroW ,olleH

Time Complexity: O(N), where N is the length of the string. 
Auxiliary Space: O(1)