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Which modification to the original two pointers approach correctly handles this scenario?

hard🎤 Interviewer Follow-up Q15 of Q15
Intervals - Interval List Intersections
Suppose the interval lists can contain intervals with negative numbers and intervals within the same list may overlap. Which modification to the original two pointers approach correctly handles this scenario?
APreprocess each list by merging overlapping intervals before applying the two pointers intersection algorithm.
BModify the two pointers algorithm to skip intervals with negative numbers, as they cannot intersect positively.
CUse the brute force nested loop approach since two pointers rely on non-overlapping intervals within each list.
DSort the combined intervals from both lists and then find intersections by scanning sequentially.
Step-by-Step Solution

  1. Step 1: Identify problem with overlapping intervals

    Two pointers assume each list is sorted and non-overlapping internally; overlapping intervals break this assumption.

  2. Step 2: Preprocess by merging intervals

    Merging each list's overlapping intervals produces sorted, non-overlapping lists suitable for two pointers.

  3. Step 3: Apply two pointers intersection

    After preprocessing, the original algorithm works correctly even with negative numbers.

  4. Final Answer:

    Option A -> Option A

  5. Quick Check:

    Preprocessing restores assumptions needed for efficient intersection [OK]
Quick Trick: Merge intervals first to restore non-overlapping property [OK]
Common Mistakes:
MISTAKES
  • Ignoring internal overlaps
  • Skipping negative intervals incorrectly
  • Switching to brute force unnecessarily
Trap Explanation:
PITFALL
  • Two pointers fail if intervals overlap internally; brute force is correct but inefficient; skipping negatives is incorrect.
Interviewer Note:
CONTEXT
  • Tests candidate's ability to adapt algorithm assumptions to relaxed constraints.
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