Given the following iterative code to generate all combinations of size k from 1 to n, what is the output of combine(4, 2)?

easy🧾 Code Trace Q3 of Q15

Subsets & Combinations - Combinations (Choose K from N)

Given the following iterative code to generate all combinations of size k from 1 to n, what is the output of combine(4, 2)?

A[[1, 2], [2, 3], [3, 4]]

B[[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]

C[[1, 2], [1, 3], [2, 3], [2, 4], [3, 4]]

D[[1, 2], [1, 3], [1, 4], [2, 3], [3, 4]]

Step-by-Step Solution

Solution:

Step 1: Trace initial combination
Start with [1, 2], add to result.
Step 2: Increment rightmost element possible
Generate [1, 3], [1, 4], then increment first element to 2 and reset second: [2, 3], [2, 4], finally [3, 4].
Final Answer:
Option B -> Option B
Quick Check:
All 6 combinations of 4 choose 2 are generated [OK]

Quick Trick: Iterative lex order generates all combinations in ascending order [OK]

Common Mistakes:

MISTAKES

Trap Explanation:

PITFALL

Candidates often miss resetting elements after increment causing missing combos.

Interviewer Note:

CONTEXT

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