An inverted alphabet right-angled triangle prints the longest letter run first, then shrinks by one letter each row — still starting from A every time.
Remember
Rule: for end letter i from last down to A,
print A through i
ABCDE
ABCD
ABC
AB
A ← 5 rows (longest on top)
It is the flip of Program 1: keep the same inner A..i print, reverse only the outer loop. That mirrors how star Program 2 inverts Program 1. Compare with Program 6, which shrinks from the left instead.
Approach
How to Solve It
Two ways to emit the same shape — start with nested char loops, then optionally shorten with Substring.
Method
Idea
Best for
Nested char loops
Outer = shrinking end; inner = A..end
Learning, interviews, exams
letters.Substring(0, i)
Slice shorter prefixes while i counts down
Shorter demos once loops click
Pseudocode
Pseudocode
for end from lastLetter down to 'A':
for ch from 'A' to end:
print ch (no newline)
print newline
Cheat sheet
Goal
Pattern
Countdown end letter
for (char i = last; i >= 'A'; i--)
Print A..i
for (char j = 'A'; j <= i; j++) Console.Write(j);
End the row
Console.WriteLine();
Top letter from rows
char top = (char)('A' + rows - 1);
One-line row shortcut
Console.WriteLine(letters.Substring(0, i)); while i shrinks
Change the row count and the inverted triangle updates instantly — capped at 26 letters (A–Z).
Whole numbers from 1 to 26. Tap a chip or type a value — the preview redraws as you go.
Live result5 rows · 15 letters
ABCDE
ABCD
ABC
AB
A
Trace
Worked Walkthrough — rows = 4
Trace each outer-loop end letter as i counts down from 'D' to 'A'.
End i
Inner j
Printed row
Letters
'D'
A..D
ABCD
4
'C'
A..C
ABC
3
'B'
A..B
AB
2
'A'
A..A
A
1
Total letter prints: 4 + 3 + 2 + 1 = 10 = 4×5/2 — same as Program 1, only the print order of rows differs.
Code
C# Programs
Three complete programs: fixed top letter, console input, and a Substring shortcut. Use View Output to reveal sample results.
Example 1 — Fixed from 'E' down to 'A'
Hard-coded top letter — outer loop shrinks; inner loop still prints A..i.
C#
using System;
class Program
{
static void Main()
{
for (char i = 'E'; i >= 'A'; i--)
{
for (char j = 'A'; j <= i; j++)
{
Console.Write(j);
}
Console.WriteLine();
}
}
}
Output
ABCDE
ABCD
ABC
AB
A
How It Works
1. Outer loop picks the end letter.i runs from 'E' down to 'A' — longest row first.
2. Inner loop restarts at A. For each i, j runs from 'A' to i, so the row is A..i.
3. Print letters, then break the line.Console.Write(j) stays on the row; WriteLine() after the inner loop starts the next (shorter) row.
When i = 'E' you get ABCDE; when i = 'A' you get A.
Example 2 — User Input Version
Read the row count at runtime. Prefer int.TryParse and clamp to 26 (shown in the tip below).
C#
using System;
class Program
{
static void Main()
{
Console.Write("Enter the number of rows: ");
int rows = Convert.ToInt32(Console.ReadLine());
char top = (char)('A' + rows - 1);
for (char i = top; i >= 'A'; i--)
{
for (char j = 'A'; j <= i; j++)
{
Console.Write(j);
}
Console.WriteLine();
}
}
}
Output (when user enters 4)
Enter the number of rows: 4
ABCD
ABC
AB
A
How It Works
1. Prompt and read. Ask for a row count, then convert the line to an int.
2. Map rows to a top letter.top = (char)('A' + rows - 1) — for rows = 4, top is 'D'.
3. Same shrink core. Only the source of top changes — the print logic matches Example 1.
4. Safer input tip.Convert.ToInt32 throws on letters. Prefer:
Safer input
if (!int.TryParse(Console.ReadLine(), out int rows) || rows < 1 || rows > 26)
{
Console.WriteLine("Enter a whole number from 1 to 26.");
return;
}
Example 3 — Substring(0, i)
Build each shrinking prefix in one call — same shape, no explicit inner letter loop.
C#
using System;
class Program
{
static void Main()
{
int rows = 5;
string letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
for (int i = rows; i >= 1; i--)
{
Console.WriteLine(letters.Substring(0, i));
}
}
}
Output
ABCDE
ABCD
ABC
AB
A
How It Works
1. Count length down.i runs from rows down to 1.
2. Slice the prefix.Substring(0, i) is ABCDE, then ABCD, and so on.
3. Learn loops first. Use Examples 1–2 when you need to show nested bounds; treat this as a polish shortcut afterward.
Edge Cases & Pitfalls
Check these before calling the solution done.
i++
Growing triangle by mistake
If you increment i from 'A' to top, you reprint Program 1. Use i-- from top down to 'A'.
Stop at B
Missing final A
Stopping before 'A' drops the tip row. The condition must be i >= 'A'.
WriteLine inside
Column of letters
If WriteLine is inside the inner loop, each letter lands on its own line. Use Write for letters; WriteLine only after the inner loop.
rows > 26
Past Z
'A' + rows - 1 leaves A–Z when rows > 26. Clamp or reject in interactive programs.
rows = 1
Single A
Output is just A — longest and shortest coincide. A good sanity check.
Bad input
Use TryParse
Convert.ToInt32 throws on letters — prefer int.TryParse and require 1–26.
Analysis
Time and Space Complexity
Program
Time
Extra space
Nested loops (Examples 1–2)
O(rows²)
O(1)
Substring (Example 3)
O(rows²)
O(rows) per temporary row string
Total letters = n + (n - 1) + … + 1 = n(n + 1)/2 — still quadratic in n. Same totals as Program 1.
Remember
Key Takeaways
Rule: countdown end letter i; print A through i each row.
Flip of Program 1: same inner loop — only reverse the outer loop.
Break the row: call WriteLine only after the inner loop.
Complexity:O(n²) time from the triangular letter count; O(1) extra space for nested loops.
One line: for i from the top letter down to 'A', print A through i, then WriteLine.
Frequently Asked Questions
The outer loop counts down so the first row prints the most letters and each next row prints one fewer. The inner loop still prints from A up to the current bound.
Because the inner loop always starts at A. That resets the sequence each row, producing ABCDE then ABCD and so on.
Program 1 grows row length (A, AB, ABC). This pattern shrinks it (ABCDE, ABCD, ABC). Only the outer loop direction changes; the inner loop is the same A..i print.
Program 4 grows while printing backward (A, BA, CBA). This pattern shrinks while printing forward from A (ABCDE, ABCD, ABC).
Console.Write stays on the same line. Console.WriteLine ends the current line. Letters use Write; the row break uses WriteLine after the inner loop.
O(n²) for n rows, because total printed characters are n(n+1)/2.
Yes. Keep a string of A–Z and print letters.Substring(0, i) while i counts down from rows to 1. Nested char loops are better for learning; Substring is a handy shortcut later.
Prefer int.TryParse(Console.ReadLine(), out rows) and clamp rows between 1 and 26 so bad input does not throw FormatException or walk past Z.
🤔
Did you know?
This is the inverted twin of Program 1: same inner A..i print, only the outer loop shrinks. For 5 rows: ABCDE, ABCD, ABC, AB, A. Total letters still equal n(n+1)/2.
