PPractick

The problem

Pattern questions ("print this triangle of stars or numbers") look like puzzles, but they're really a test of whether you can translate a picture into loop bounds. Instead of memorising 22 separate patterns, you want one method that solves all of them: (1) the outer loop counts rows, (2) for each row, work out how many of each character appear as a formula in the row number, (3) print them with inner loops, (4) look for symmetry. Apply the method to a medium pattern, a centred pyramid where each row counts up to the row number and back down again.

Example for n = 4:

   1
  121
 12321
1234321

Function signature

vector<string> numberPyramid(int n);  // one string per row, leading spaces included, no trailing spaces

Test cases

Input n	Output rows	Explanation
1	1	Single row, no leading spaces
2	" 1", "121"	Row 1 has one leading space
3	" 1", " 121", "12321"	Leading spaces shrink by one per row
4	" 1", " 121", " 12321", "1234321"	Row i has n − i spaces, then 1..i, then i − 1..1

(Confirm: 1 ≤ n ≤ 9 so every number is a single digit and the pyramid stays aligned; rows have no trailing spaces.)

How interviewers phrase it

• "Print this pattern for a given N," usually drawn on the whiteboard for N = 4 or 5.
• Common in campus placements and first-round screens, especially for freshers, because it tests loop control without any data structures.
• Follow-up variants: "Now print it upside down," "Now make it hollow," "Now use letters instead of numbers."

Clarifying questions to ask

1. What's the range of N? (Two-digit numbers break the alignment.)
2. Should rows have trailing spaces, or should each row end right after the last character?
3. Should I print the pattern directly or return the rows?
4. Is the pattern 1-indexed as drawn (the first row has one number)?

Follow-ups to expect

1. "How do you approach any pattern problem?"
2. "How would you print it upside down?"
3. "What changes for a diamond?"
4. "What if N can be 12?"
5. "What's the complexity?"

Related questions

• Hard Easy
• Pattern 1 Easy
• Pattern 2 Easy
• Pattern 3 Easy
• Pattern 4 Easy
• Pattern 5 Easy

All 474 dsa interview questions

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