PPractick

The problem

This pattern is a sideways triangle: the rows grow from one star to n stars, then shrink back down to one. It's a right-angled triangle (Pattern 2) followed by an inverted one (Pattern 5) that shares the widest row, so there are 2n − 1 rows in total. The interesting part is that it can be written with a single loop over all rows if you can express "how many stars in row r" as one formula that rises and then falls.

Example for n = 3:

*
**
***
**
*

Function signature

vector<string> halfDiamond(int n);  // 2n - 1 rows: 1, 2, ..., n, ..., 2, 1 stars

Test cases

Input n	Output rows	Explanation
1	"*"	A single row
2	"*", "**", "*"	Rises to 2 and falls back
3	"*", "**", "***", "**", "*"	Widest row appears once
4	1, 2, 3, 4, 3, 2, 1 stars	2n − 1 = 7 rows

(Confirm: the widest row appears exactly once; rows are left-aligned with no separators.)

How interviewers phrase it

• "Print a half diamond (right-pointing arrow) of stars."
• "Print stars increasing from 1 to N and back down to 1, one row per count."

Clarifying questions to ask

1. Should the widest row appear once or twice?
2. Left-aligned?
3. Print or return?

Follow-ups to expect

1. "Can you do it with one loop?"
2. "How do you avoid printing the middle row twice?"
3. "What's the complexity?"

Related questions

• Easy and Medium Easy
• Hard Easy
• Pattern 1 Easy
• Pattern 2 Easy
• Pattern 3 Easy
• Pattern 4 Easy

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