PPractick

The problem

The butterfly is the mirror image of the symmetric void (Pattern 19): instead of a hole that opens and closes, two "wings" of stars grow toward the middle and shrink away again, while the gap between them does the opposite. Each row still has three segments (stars, gap, stars), and every row is 2n characters wide. The widest row, where the wings touch, appears once, so there are 2n − 1 rows.

Example for n = 5:

*        *
**      **
***    ***
****  ****
**********
****  ****
***    ***
**      **
*        *

Function signature

vector<string> butterfly(int n);  // 2n - 1 rows, each 2n characters wide

Test cases

Input n	Output rows	Explanation
1	"**"	One row, the wings already touch
2	"* *", "****", "* *"	Gap 2, then 0, then 2
3	"* *", "** **", "******", "** **", "* *"	Stars grow 1, 2, 3 then shrink
5	as in the example	9 rows of 10 characters

(Confirm: the widest row appears once, giving 2n − 1 rows; rows end with a star, so there are no trailing spaces.)

How interviewers phrase it

• "Print the butterfly pattern for N."
• Often asked right after the symmetric void: "Now invert it."

Clarifying questions to ask

1. Should the middle (full) row appear once or twice?
2. Is N the number of stars on each side of the widest row?
3. Print or return?

Follow-ups to expect

1. "What's the star count per side in row r?"
2. "And the gap?"
3. "How is it related to Pattern 19?"
4. "Complexity?"

Related questions

• Pattern 21 Medium
• Pattern 22 Medium
• Easy and Medium Easy
• Hard Easy
• Pattern 1 Easy
• Pattern 2 Easy

All 474 dsa interview questions

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