[Solved] Consider the following buggy code for inserting an interval. Which line contains the subtle bug... Which line contains the subtle bug that can cause incorrect output when the input list is empty? | Intervals

Intervals - Insert Interval

Consider the following buggy code for inserting an interval. Which line contains the subtle bug that can cause incorrect output when the input list is empty?

from typing import List

def insert(intervals: List[List[int]], newInterval: List[int]) -> List[List[int]]:
    intervals.append(newInterval)
    intervals.sort(key=lambda x: x[0])
    merged = [intervals[0]]
    for i in range(1, len(intervals)):
        if intervals[i][0] <= merged[-1][1]:
            merged[-1][1] = max(merged[-1][1], intervals[i][1])
        else:
            merged.append(intervals[i])
    return merged

ALine 5: merged = [intervals[0]]

BLine 3: intervals.append(newInterval)

CLine 7: if intervals[i][0] <= merged[-1][1]:

DLine 9: merged.append(intervals[i])

Step-by-Step Solution

Solution:

Step 1: Consider empty intervals input
If intervals is empty, after appending newInterval, intervals has length 1.
Step 2: Check initialization of merged
Line 5 accesses intervals[0] without checking if intervals is empty before append, which is safe here but if input was empty, intervals[0] is newInterval, so no crash.
Step 3: Identify subtle bug
Actually, no crash here, but if input intervals was empty, sorting and merging still works. The subtle bug is that if intervals was empty and newInterval is appended, the code works, but if the initial intervals list is empty and the code did not append newInterval first, line 5 would crash. So the bug is that the code assumes intervals is non-empty before append, which is not always true.
Final Answer:
Option A -> Option A
Quick Check:
Accessing intervals[0] without empty check is risky [OK]

Quick Trick: Accessing intervals[0] without empty check causes bug [OK]

Common Mistakes:

MISTAKES

Not handling empty input intervals list
Assuming intervals always non-empty before append
Incorrect merging when intervals just touch

Trap Explanation:

PITFALL

Option B looks like normal initialization but fails on empty input; others are correct lines.

Interviewer Note:

CONTEXT

Tests if candidate spots subtle assumptions about input validity and empty lists.

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More Intervals Quizzes

Consider the following buggy code for inserting an interval. Which line contains the subtle bug that can cause incorrect output when the input list is empty?

Step 1: Consider empty intervals input

Step 2: Check initialization of merged

Step 3: Identify subtle bug

Final Answer:

Quick Check:

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