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Reversing a String in C

In this article, we will discuss how to implement a function in C to reverse a given string. We will explore two approaches: one using a temporary array and another using a two-pointer technique.

Approach 1: Using a Temporary Array

This approach involves creating a temporary array to store the characters of the input string in reverse order. We will then copy the characters from the temporary array back to the original string.


// Function to reverse a string using a temporary array
void reverse_string_temp(char *str) {
    int length = strlen(str);
    char temp[length + 1];

    // Copy characters from the original string to the temporary array in reverse order
    for (int i = 0; i < length; i++) {
        temp[i] = str[length - i - 1];
    }
    temp[length] = '\0'; // Null-terminate the temporary array

    // Copy characters from the temporary array back to the original string
    strcpy(str, temp);
}

Approach 2: Using a Two-Pointer Technique

This approach involves using two pointers, one starting from the beginning of the string and the other from the end. We will swap the characters at the positions pointed to by the two pointers and move the pointers towards each other.


// Function to reverse a string using a two-pointer technique
void reverse_string_two_pointers(char *str) {
    int length = strlen(str);
    char *start = str;
    char *end = str + length - 1;

    while (start < end) {
        // Swap characters at the positions pointed to by the two pointers
        char temp = *start;
        *start = *end;
        *end = temp;

        // Move the pointers towards each other
        start++;
        end--;
    }
}

Example Usage

Here's an example of how to use the `reverse_string_temp` and `reverse_string_two_pointers` functions:


int main() {
    char str[] = "Hello, World!";

    printf("Original string: %s\n", str);

    // Reverse the string using the temporary array approach
    reverse_string_temp(str);
    printf("Reversed string (temp array): %s\n", str);

    // Reverse the string using the two-pointer technique
    reverse_string_two_pointers(str);
    printf("Reversed string (two pointers): %s\n", str);

    return 0;
}

Output

The output of the example program will be:


Original string: Hello, World!
Reversed string (temp array): !dlroW ,olleH
Reversed string (two pointers): !dlroW ,olleH

Conclusion

In this article, we discussed two approaches to reversing a string in C: using a temporary array and using a two-pointer technique. Both approaches have their own advantages and disadvantages. The temporary array approach is simpler to implement but requires extra memory, while the two-pointer technique is more efficient but requires more complex logic.

FAQs

Q: What is the time complexity of the `reverse_string_temp` function?

A: The time complexity of the `reverse_string_temp` function is O(n), where n is the length of the input string.

Q: What is the space complexity of the `reverse_string_temp` function?

A: The space complexity of the `reverse_string_temp` function is O(n), where n is the length of the input string.

Q: What is the time complexity of the `reverse_string_two_pointers` function?

A: The time complexity of the `reverse_string_two_pointers` function is O(n/2), which simplifies to O(n), where n is the length of the input string.

Q: What is the space complexity of the `reverse_string_two_pointers` function?

A: The space complexity of the `reverse_string_two_pointers` function is O(1), as it only uses a constant amount of extra memory.

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