C# Program to Check Palindrome String

You can check a palindrome string in C# by comparing the original string with its reversed version using string reversed = new string(input.Reverse().ToArray()); and then checking input == reversed.

Examples

Inputmadam

Outputmadam is a palindrome.

Inputhello

Outputhello is not a palindrome.

InputA

OutputA is a palindrome.

How to Think About It

To check if a string is a palindrome, think of reading it from left to right and right to left. If both readings are exactly the same, the string is a palindrome. So, reverse the string and compare it with the original. If they match, it is a palindrome; otherwise, it is not.

Algorithm

1

Get the input string.

2

Reverse the input string.

3

Compare the original string with the reversed string.

4

If both are equal, return that the string is a palindrome.

5

Otherwise, return that it is not a palindrome.

Code

csharp

using System;
using System.Linq;

class Program {
    static void Main() {
        Console.Write("Enter a string: ");
        string input = Console.ReadLine();
        string reversed = new string(input.Reverse().ToArray());
        if (input == reversed) {
            Console.WriteLine($"{input} is a palindrome.");
        } else {
            Console.WriteLine($"{input} is not a palindrome.");
        }
    }
}

Output

Enter a string: madam
madam is a palindrome.

Dry Run

Let's trace the input 'madam' through the code.

1

Input string

input = 'madam'

2

Reverse string

reversed = 'madam'

3

Compare strings

input == reversed -> 'madam' == 'madam' is true

4

Output result

Print 'madam is a palindrome.'

Step	input	reversed	Comparison Result
1	madam
2	madam	madam
3	madam	madam	true

Why This Works

Step 1: Reverse the string

We use input.Reverse() to get characters in reverse order and create a new string from them.

Step 2: Compare original and reversed

If the original string and reversed string are exactly the same, it means the string reads the same forwards and backwards.

Step 3: Print result

Based on the comparison, we print whether the string is a palindrome or not.

Alternative Approaches

Two-pointer approach

csharp

using System;

class Program {
    static bool IsPalindrome(string s) {
        int left = 0, right = s.Length - 1;
        while (left < right) {
            if (s[left] != s[right]) return false;
            left++;
            right--;
        }
        return true;
    }

    static void Main() {
        Console.Write("Enter a string: ");
        string input = Console.ReadLine();
        if (IsPalindrome(input)) {
            Console.WriteLine($"{input} is a palindrome.");
        } else {
            Console.WriteLine($"{input} is not a palindrome.");
        }
    }
}

This method checks characters from both ends moving inward without creating a new string, saving memory.

Using for loop to build reversed string

csharp

using System;

class Program {
    static void Main() {
        Console.Write("Enter a string: ");
        string input = Console.ReadLine();
        string reversed = "";
        for (int i = input.Length - 1; i >= 0; i--) {
            reversed += input[i];
        }
        if (input == reversed) {
            Console.WriteLine($"{input} is a palindrome.");
        } else {
            Console.WriteLine($"{input} is not a palindrome.");
        }
    }
}

This builds the reversed string manually but is less efficient due to string concatenation in a loop.

Complexity: O(n) time, O(n) space

Time Complexity

Reversing the string takes O(n) time where n is the string length, and comparing two strings also takes O(n). Overall complexity is O(n).

Space Complexity

Creating a reversed string requires O(n) extra space. The two-pointer approach can reduce space to O(1).

Which Approach is Fastest?

The two-pointer approach is fastest in space as it avoids extra string creation, but using Reverse() is simpler and clear for beginners.

Approach	Time	Space	Best For
Using string.Reverse()	O(n)	O(n)	Simplicity and readability
Two-pointer approach	O(n)	O(1)	Memory efficiency
Manual for loop reversal	O(n)	O(n)	Understanding string manipulation basics