How to reverse a string in C and C#?

The fastest way to reverse a string in C is an in-place two-pointer swap: one pointer starts at the first character, one at the last, and you swap until they meet. In C#, the simplest way is Array.Reverse() on a character array. Both run in O(n) time.

This answer shows the standard methods in both languages, with code you can use directly in a coding interview.

How to Reverse a String in C

1. Two-pointer swap (in place)

This is the classic C solution and the one interviewers expect. It uses O(1) extra space.

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

void reverseString(char str[]) {
    int left = 0;
    int right = strlen(str) - 1;
    while (left < right) {
        char temp = str[left];
        str[left] = str[right];
        str[right] = temp;
        left++;
        right--;
    }
}

int main(void) {
    char text[] = "Hello, World!";
    reverseString(text);
    printf("%s\n", text);   // !dlroW ,olleH
    return 0;
}

2. Loop into a new buffer

If you must keep the original string unchanged, copy characters back to front into a second array.

void reverseCopy(const char *src, char *dest) {
    int len = strlen(src);
    for (int i = 0; i < len; i++) {
        dest[i] = src[len - 1 - i];
    }
    dest[len] = '\0';
}

3. Recursion

Recursion works but costs a stack frame per character, so use it only when asked.

void reverseRecursive(char str[], int left, int right) {
    if (left >= right) return;
    char temp = str[left];
    str[left] = str[right];
    str[right] = temp;
    reverseRecursive(str, left + 1, right - 1);
}

A note on strrev(): it exists in some compilers (Turbo C, some MSVC versions) but it is not part of the C standard, so portable code and interview answers should use the two-pointer swap instead.

How to Reverse a String in C#

Strings in C# are immutable, so every method builds a new string.

1. Using Array.Reverse() (recommended)

public string ReverseString(string input)
{
    char[] charArray = input.ToCharArray();
    Array.Reverse(charArray);
    return new string(charArray);
}

2. Using a for loop

public string ReverseString(string input)
{
    string reversed = string.Empty;
    for (int i = input.Length - 1; i >= 0; i--)
    {
        reversed += input[i];
    }
    return reversed;
}

Avoid this for long strings: each += allocates a new string, making it O(n^2).

3. Using LINQ

using System.Linq;

public string ReverseString(string input)
{
    return new string(input.Reverse().ToArray());
}

4. Using StringBuilder

using System.Text;

public string ReverseString(string input)
{
    StringBuilder reversed = new StringBuilder(input.Length);
    for (int i = input.Length - 1; i >= 0; i--)
    {
        reversed.Append(input[i]);
    }
    return reversed.ToString();
}

Which method should you use?

Frequently Asked Questions

How do you reverse a string in Python or Java?

In Python it is one line: reversed_string = s[::-1]. In Java, use new StringBuilder(s).reverse().toString(). The underlying idea is the same two-pointer reversal.

Why does the interviewer care which method I pick?

String reversal is usually a warm-up to test whether you know about immutability, in-place operations, and time/space complexity. Mentioning that the two-pointer swap is O(n) time and O(1) space is what earns the follow-up question.

Does this handle Unicode correctly?

Byte-by-byte or char-by-char reversal breaks multi-byte characters (emoji, combining accents). For interview problems ASCII is assumed; in production, reverse by grapheme clusters using your platform's text APIs.

String reversal is one of the most common warm-up questions in coding interviews. If you are preparing seriously, the two-pointer technique behind it is one of the core patterns in Grokking the Coding Interview, and Grokking Data Structures covers the array and string fundamentals these questions build on.