Why does the second loop start from i-1?

Starting from i-1 avoids repeating the middle number. For example, for i=3, printing 123 then 21 makes 12321.

How do I add spaces between numbers?

Use Console.Write(j + " ") and Console.Write(k + " ") in the loops instead of Console.Write(j).

What is the time complexity?

O(n^2) for n rows because each row prints O(i) digits and total prints are proportional to 1+2+...+n.

Palindrome Number Triangle in C#

Beginner

⏱️ 5 min read

📚 Updated: Aug 2025

🎯 2 Code Examples

Mirroring

Palindrome number triangle output (1, 121, 12321, 1234321, 123454321) in C# programming

What You’ll Learn

How to build a palindrome triangle in C# by printing an ascending sequence and then mirroring it back down on the same row.

⭐ Pattern Output

For rows = 5, the pattern looks like this:

Output

Complete C# Program

Print 1..i first, then print i-1..1 to mirror the row.

using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            int i, j, k;

            for (i = 1; i <= 5; i++)
            {
                for (j = 1; j <= i; j++)
                    Console.Write(j);

                for (k = i - 1; k >= 1; k--)
                    Console.Write(k);

                Console.WriteLine();
            }
        }
    }
}

🧠 How It Works

Outer loop controls rows

i decides how far the sequence grows on each line.

Rows

Print ascending part

The first inner loop prints 1..i.

Left half

Mirror back down

The second loop prints i-1..1, creating a palindrome.

Right half

1
121
12321

Palindrome rows

Each row reads the same left-to-right and right-to-left.

Variation — User Input Rows

Let the user choose how many rows to print:

using System;

namespace MyApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Enter the number of rows: ");
            int rows = Convert.ToInt32(Console.ReadLine());

            for (int i = 1; i <= rows; i++)
            {
                for (int j = 1; j <= i; j++)
                    Console.Write(j);

                for (int k = i - 1; k >= 1; k--)
                    Console.Write(k);

                Console.WriteLine();
            }
        }
    }
}