PPractick

The problem

The hollow square draws only the border of an n × n square: the first and last rows are solid, and every row in between has a star at each end with spaces inside. It's the first pattern where the character at a position depends on a condition (is this cell on the border?) rather than a simple count, which is exactly how grid problems check boundaries later on.

Example for n = 4:

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

Function signature

vector<string> hollowSquare(int n);  // n rows of n characters; stars on the border only

Test cases

Input n	Output rows	Explanation
1	"*"	A single cell is all border
2	"**", "**"	Every cell is on the border
3	"***", "* *", "***"	One interior cell
4	"****", "* *", "* *", "****"	A 2 × 2 interior

(Confirm: the pattern is an n × n square; interior rows end with a star, so there are no trailing spaces.)

How interviewers phrase it

• "Print a hollow square (or rectangle) of stars."
• "Print only the border of an N × N grid."

Clarifying questions to ask

1. Square (n × n) or rectangle (r × c)?
2. Stars separated by spaces?
3. What should n = 1 and n = 2 look like?

Follow-ups to expect

1. "What's the condition for a border cell?"
2. "How would you do a hollow rectangle of r rows and c columns?"
3. "How would you draw the diagonals too?"
4. "Complexity?"

Related questions

• Pattern 20 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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