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Consider the following Python code implementing the optimal backtracking solution for Combination Sum III. What is the final value of res after calling combinationSum3(3, 7)?

easy🧾 Code Trace Q12 of Q15

Subsets & Combinations - Combination Sum III (K Numbers to N)

Consider the following Python code implementing the optimal backtracking solution for Combination Sum III. What is the final value of res after calling combinationSum3(3, 7)?

A[[1, 2, 4]]

B[[1, 3, 3]]

C[[2, 2, 3]]

D[[1, 2, 3]]

Step-by-Step Solution

Step 1: Trace backtracking calls for k=3, n=7
Start from 1, try combinations of size 3 summing to 7. Valid combos are [1,2,4] only because 1+2+4=7.
Step 2: Verify no other combos meet criteria
Other combos like [1,3,3] or [2,2,3] are invalid due to duplicates or sum mismatch.
Final Answer:
Option A -> Option A
Quick Check:
Only [1,2,4] sums to 7 with 3 distinct numbers [OK]

Quick Trick: Only distinct numbers summing to 7 with size 3 is [1,2,4] [OK]

Common Mistakes:

MISTAKES

Including duplicates like [1,3,3]
Miscounting sum or size

Trap Explanation:

PITFALL

Candidates often forget distinctness or miscalculate sums leading to wrong combos.

Interviewer Note:

CONTEXT

Tests candidate's ability to mentally execute backtracking and verify correctness.

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