C# Palindrome Number Triangle Pattern (Outer Peak)
Beginner
5 min read
Updated: Sep 2026
3 programs
Live preview
Definition
What Is This Pattern?
A palindrome number triangle prints digits that read the same forward and backward on each row. Build the left half by counting down, then the right half by counting up through 1.
Remember
Rule: print i..2, then 1..i (center is always 1)
1
212
32123
4321234
543212345 ← 5 rows
In C# use two inner loops per row: j from i down to 2, then j from 1 up to i. Row i prints 2i - 1 digits with no spaces between them.
Approach
How to Solve It
One outer loop and two inner loops that mirror around 1.
Method
Idea
Best for
Two-loop palindrome
Descend to 2, then ascend from 1
Learning, interviews, exams
User-input rows
Same logic with a variable height
Practice / demos
Pseudocode
Pseudocode
for i from 1 to rows:
for j from i down to 2:
print j
for j from 1 to i:
print j
print newline
Cheat sheet
Goal
Pattern
Grow the row
for (i = 1; i <= rows; i++)
Left half
for (j = i; j > 1; j--) Console.Write(j);
Right half
for (j = 1; j <= i; j++) Console.Write(j);
End the row
Console.WriteLine();
Write vs WriteLine
API
Effect
Use for
Console.Write
Stays on the same line
Each digit (no spaces)
Console.WriteLine
Ends the current line
After both inner loops
Try it
Live Preview
Change the row count and the palindrome triangle updates instantly — including total digit count.
Whole numbers from 1 to 9 (keeps digits single-width). Tap a chip or type a value — the preview redraws as you go.
Live result5 rows · 25 digits
1
212
32123
4321234
543212345
Trace
Worked Walkthrough — rows = 4
Trace the left descent and the right ascent. The center digit is always 1.
i
Left / right
Printed row
1
(none) / 1
1
2
2 / 12
212
3
32 / 123
32123
4
432 / 1234
4321234
The left loop stops at 2 so the ascending loop can print the center 1 once — never twice.
Code
C# Programs
Three complete programs: fixed height, user input, and a compact 3-row demo. Use View Output for sample results.
Example 1 — Fixed rows = 5
Two inner loops per row: descend to 2, then ascend from 1 through i.
C#
using System;
namespace MyApp
{
class Program
{
static void Main(string[] args)
{
int rows = 5;
int i, j;
for (i = 1; i <= rows; i++)
{
for (j = i; j > 1; j--)
Console.Write(j);
for (j = 1; j <= i; j++)
Console.Write(j);
Console.WriteLine();
}
}
}
}
Output
1
212
32123
4321234
543212345
How It Works
1. Outer loop.i grows from 1 to 5 — each row is a longer palindrome.
2. Left half.j runs from i down to 2 (skipped entirely when i = 1).
3. Right half.j runs from 1 to i, placing the center 1 and the ascending mirror.
Example 2 — User Input
Read the row count and apply the same two-loop palindrome for any height.
C#
using System;
namespace MyApp
{
class Program
{
static void Main(string[] args)
{
int rows;
int i, j;
Console.Write("Enter rows: ");
if (!int.TryParse(Console.ReadLine(), out rows) || rows < 1)
{
Console.WriteLine("Please enter a positive whole number.");
return;
}
for (i = 1; i <= rows; i++)
{
for (j = i; j > 1; j--)
Console.Write(j);
for (j = 1; j <= i; j++)
Console.Write(j);
Console.WriteLine();
}
}
}
}
A smaller fixed demo — same two-loop idea, easier to trace by hand.
C#
using System;
namespace MyApp
{
class Program
{
static void Main(string[] args)
{
int rows = 3;
int i, j;
for (i = 1; i <= rows; i++)
{
for (j = i; j > 1; j--)
Console.Write(j);
for (j = 1; j <= i; j++)
Console.Write(j);
Console.WriteLine();
}
}
}
}
Output
1
212
32123
How It Works
1. Same rules. Left prints i..2; right prints 1..i.
2. Quick check. The longest row is 32123 — five digits for i = 3.
Edge Cases & Pitfalls
Check these before calling the solution done.
double 1
Descend all the way to 1
Using j >= 1 on the left prints two 1s. Stop at j > 1 so the ascending loop owns the center.
WriteLine early
WriteLine between the two halves
That splits the palindrome onto two lines. Call WriteLine() only after both loops finish.
extra spaces
Print a space after each digit
This pattern is tight digits only. Use Console.Write(j) with no trailing space.
rows = 1
Single 1
The left loop does not run; only the ascending loop prints 1.
rows ≤ 0
Empty output
The outer loop never runs. Validate and prompt again for clearer UX.
Bad input
Convert.ToInt32 throws
Prefer int.TryParse so non-numeric input does not crash the program.
Analysis
Time and Space Complexity
Program
Time
Extra space
Fixed / compact (Examples 1, 3)
O(n²)
O(1)
User input (Example 2)
O(n²)
O(1)
Total digits are 1 + 3 + 5 + … + (2n-1) = n², so work is quadratic in n.
Remember
Key Takeaways
Rule: print i..2, then 1..i — the center is always 1.
Length: row i has 2i - 1 digits; all rows together print n² digits.
Write vs WriteLine: digits stay on the line; WriteLine advances after both halves.
Next step: Program 38 prints a decreasing-length triangle of continuous numbers.
One line: for each row i, print a descending left half and an ascending right half that meet at 1.
Frequently Asked Questions
Each row reads the same forward and backward. For example, 4321234 is symmetric around the center digit 1.
First, a loop prints i down to 2 (left half). Then another loop prints 1 up to i (right half). Together they create a mirrored sequence.
The first loop prints 4 3 2, and the second loop prints 1 2 3 4, which together form 4321234.
Row i prints 2i - 1 digits — one more than the previous row's digit count by 2.
Program 36 is right-aligned with decreasing sequences. Program 37 builds a symmetric palindrome on each row with two inner loops.
Replace 5 with rows in the outer loop bound — see Example 2.
O(n²) for n rows because total digits printed are 1 + 3 + 5 + ... + (2n-1) = n².
Prefer int.TryParse(Console.ReadLine(), out rows) so bad input does not throw FormatException.
Only one row prints — a single digit 1.
🤔
Did you know?
Each row is a palindrome: print i down to 2, then 1 up to i. Row i prints 2i - 1 digits — total digits across all rows = n².
