A reverse-order alphabet triangle prints a left-aligned staircase where each row’s highest letter grows forward, but letters along the row print backward down to A.
Remember
Rule: on row with peak $i, print $i down to A
A
BA
CBA
DCBA
EDCBA ← 5 rows
It is the row-order flip of Program 1: same growing widths, but the inner loop counts down instead of up. Compare with Program 3, which changes the start letter while still printing forward.
Approach
How to Solve It
Two ways to emit the same shape — start with nested letter loops, then optionally reverse a substr.
Method
Idea
Best for
Nested letter loops
Outer = peak letter; inner = peak down to A
Learning, interviews, exams
strrev(substr(...))
Slice A..peak, reverse, print once
Shorter demos once loops click
Pseudocode
Pseudocode
for $peak from 'A' to $lastLetter:
for $ch from $peak down to 'A':
echo $ch
echo PHP_EOL
1. Outer loop picks the peak.$i runs from 'A' to 'E' — one row per peak letter.
2. Inner loop counts down. For each $i, $j runs from $i down to 'A', so the row is $i..A.
3. Print letters, then break the line.echo $j stays on the row; echo PHP_EOL after the inner loop starts the next row.
When $i = 'A' you get A; when $i = 'B' you get BA; up through EDCBA.
Example 2 — User Input Version
Read the row count at runtime. Prefer validating with is_numeric and clamp to 26 (shown in the tip below).
PHP
<?php
echo "Enter the number of rows: ";
$rows = (int) trim(fgets(STDIN));
$last = chr(ord('A') + $rows - 1);
for ($i = 'A'; $i <= $last; $i++) {
for ($j = $i; $j >= 'A'; $j--) {
echo $j;
}
echo PHP_EOL;
}
Output (when user enters 4)
Enter the number of rows: 4
A
BA
CBA
DCBA
How It Works
1. Prompt and read. Ask for a row count, then cast the trimmed line to an int.
2. Map rows to a last peak.$last = chr(ord('A') + $rows - 1) — for $rows = 4, $last is 'D'.
3. Same countdown core. Only the source of $last changes — the print logic matches Example 1.
4. Safer input tip. Prefer:
Safer input
$raw = trim(fgets(STDIN));
if (!is_numeric($raw)) {
echo "Enter a whole number from 1 to 26." . PHP_EOL;
exit;
}
$rows = (int) $raw;
if ($rows < 1 || $rows > 26) {
echo "Enter a whole number from 1 to 26." . PHP_EOL;
exit;
}
Example 3 — strrev(substr(...))
Build A..peak with substr, reverse it, and print — same shape, no explicit inner countdown.
1. Slice the forward prefix.substr($letters, 0, $i) is A, AB, ABC, … — same as Program 1.
2. Reverse the slice.strrev turns that prefix into A, BA, CBA, …
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.
$j++
Forward triangle by mistake
If the inner loop increments from A to $i, you reprint Program 1. Keep $j-- from $i down to 'A'.
Stop at B
Missing final A
Stopping before 'A' drops the tip letter every row. The condition must be $j >= 'A'.
PHP_EOL inside
Column of letters
If echo PHP_EOL is inside the inner loop, each letter lands on its own line. Use echo for letters; break the row only after the inner loop.
$rows > 26
Past Z
chr(ord('A') + $rows - 1) leaves A–Z when $rows > 26. Clamp or reject in interactive programs.
$rows = 1
Single A
Output is just A — a good sanity check.
Bad input
Use is_numeric
Bare (int) fgets(STDIN) turns letters into 0 — validate before the outer loop.
Analysis
Time and Space Complexity
Program
Time
Extra space
Nested loops (Examples 1–2)
O(rows²)
O(1)
strrev(substr) (Example 3)
O(rows²)
O(rows) per temporary row string
Total letters = 1 + 2 + … + n = n(n + 1)/2 — still quadratic in n. Same totals as Program 1; only print order along each row differs.
Remember
Key Takeaways
Rule: peak letter grows A..$last; print that peak down to A.
Flip of Program 1: same widths — reverse the inner loop direction.
Break the row: call echo PHP_EOL only after the inner loop.
Complexity:O(n²) time; O(1) extra space for nested loops.
One line: for each peak $i from 'A' to the last letter, print $i down to 'A', then echo PHP_EOL.
Frequently Asked Questions
The outer loop selects the highest letter on the row (A to E). The inner loop starts at that letter and decrements down to A, printing each character, so each row reads backward.
Because the inner loop always runs until $j equals A, so the last printed character on every row is A.
Program 1 prints forward along each row (A, AB, ABC). This pattern prints backward down to A (A, BA, CBA) while the left edge still grows A, B, C, …
Program 3 starts earlier each row but still prints forward to a fixed top (E, DE, CDE). This pattern starts later each row and prints backward to a fixed A.
echo stays on the same line. echo PHP_EOL ends the current line. Letters use echo; the row break uses echo PHP_EOL after the inner loop.
O(n²) for n rows, because the total printed letters are 1+2+…+n = n(n+1)/2.
Yes. Take substr($letters, 0, $i), reverse with strrev, and echo the result with PHP_EOL. Nested letter loops are better for learning; strrev is a handy shortcut later.
Use trim(fgets(STDIN)) and check is_numeric($input) before casting to int, then clamp rows between 1 and 26 so bad input does not walk past Z.
🤔
Did you know?
Each row starts one letter later (A, B, C, …), but prints backward down to A. For 5 rows, the output is A, BA, CBA, DCBA, EDCBA — the reverse of Program 1’s row order.
