LeetCode 3709: Design Exam Scores Tracker ·

Table of Contents

Meta Data
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Difficulty: medium First Attempt: 2025-10-11

Submissions: Wrong Answer → Runtime Error → Wrong Answer → Accepted

Approach
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record times are strictly increasing, so the time array is already sorted.

times: the time of every record.
prefix: prefix sums of scores, where prefix[i] is the total of the first i records and prefix[0] = 0.

For a query [start, end]:

left  = first position with time >= start = bisect_left(times, start)
right = first position with time >  end   = bisect_right(times, end)
total = prefix[right] - prefix[left]

The leading 0 in prefix removes the special case where the left boundary is 0.

My Attempts
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WA and RE: `mid` without parentheses
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mid = left + right // 2       # means left + (right // 2)

// binds tighter than +, so mid overshoots: sometimes it finds the wrong position (WA), sometimes it goes out of range (RE).

WA: a query range with no records
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After fixing mid, a range with no exams produced a left boundary larger than the right one, giving negative totals or bad indices. Several boundary checks were added before it passed.

The root cause was a prefix array without a leading 0, which forced special cases for “is the left boundary 0”. With prefix[0] = 0 and two half-open boundaries, none of those checks are needed: with no records the two boundaries are equal, so the difference is 0.

Solution
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import bisect

class ExamTracker:

    def __init__(self):
        self.times = []
        self.prefix = [0]

    def record(self, time: int, score: int) -> None:
        self.times.append(time)
        self.prefix.append(self.prefix[-1] + score)

    def totalScore(self, startTime: int, endTime: int) -> int:
        lo = bisect.bisect_left(self.times, startTime)
        hi = bisect.bisect_right(self.times, endTime)
        return self.prefix[hi] - self.prefix[lo]

Complexity
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record is O(1), totalScore is O(log n). Space O(n).

Mistakes to Avoid
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mid = left + right // 2: missing parentheses.
A prefix array without a leading 0, leading to many special cases.
Not returning 0 when the range has no records.

Interview Explanation
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Record times are strictly increasing, so I store the times and prefix sums of scores, with a leading 0 in the prefix. A query uses bisect_left for the first time at or after the start and bisect_right for the first time after the end; the prefix difference is the total, and with no records the two positions coincide, giving 0. Records are O(1), queries O(log n).