PPractick

The problem

The number crown has numbers climbing up from the left edge and back down on the right, with a gap in the middle that closes row by row. Each row has three segments: count up 1..i, a gap of 2(n − i) spaces, then count down i..1. The last row has no gap at all, so the two halves meet. It's good practice for patterns whose middle segment shrinks while the outer segments grow.

Example for n = 4:

1      1
12    21
123  321
12344321

Function signature

vector<string> numberCrown(int n);  // row i: 1..i, 2(n - i) spaces, i..1

Test cases

Input n	Output rows	Explanation
1	"11"	No gap in the only row
2	"1 1", "1221"	Gap of 2, then none
3	"1 1", "12 21", "123321"	Gap shrinks by 2 each row
4	as in the example	Every row is 2n characters wide

(Confirm: 1 ≤ n ≤ 9 so all numbers are single digits; no separators between digits.)

How interviewers phrase it

• "Print the number crown pattern for N."
• "Print numbers ascending on the left and descending on the right with spaces in between."

Clarifying questions to ask

1. Is N at most 9? (Multi-digit numbers break alignment.)
2. Should digits be separated by spaces?
3. Print or return?

Follow-ups to expect

1. "How did you get 2(n − i) spaces?"
2. "Why does the last row have no gap?"
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

All 474 dsa interview questions

DSA mock interviews · System design interview questions · Free mock interview practice · Browse interviewers · Sign in