Count Failing, Passing and Distinction Scores from Percentage Thresholds
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Online Assessment
You are given an array of integers `scores`, where the index is a student ID and the value is that student's exam score as a percentage. Each student is graded as follows:
- below 50: **failed**,
- from 50 to 80 inclusive: **passed**,
- above 80: **passed with distinction**.
Return an array of three integers: the number of students who failed, the number who passed, and the number who passed with distinction, in that order.
### Function Signature
```python
def grade_counts(scores: list[int]) -> list[int]:
```
### Rules
- A score of exactly 50 or exactly 80 counts as passed; a score of exactly 81 counts as passed with distinction.
- Every student falls into exactly one of the three groups, so the three counts sum to `len(scores)`.
- An empty array returns `[0, 0, 0]`.
### Constraints
- `0 <= len(scores) <= 100000`
- `0 <= scores[i] <= 100`
### Examples
**Example 1**
- Input: `scores = [30, 50, 65, 80, 81, 95, 49]`
- Output: `[2, 3, 2]`
- Explanation: 30 and 49 fail; 50, 65 and 80 pass; 81 and 95 pass with distinction.
**Example 2**
- Input: `scores = [100, 100]`
- Output: `[0, 0, 2]`
- Explanation: Both students scored above 80.
**Example 3**
- Input: `scores = []`
- Output: `[0, 0, 0]`
- Explanation: With no students, every count is zero.
Overview: Given an array of student exam scores as percentages, count how many students failed with under 50, passed with 50 to 80 inclusive, or passed with distinction above 80, and return the three counts in order. It tests careful handling of inclusive and exclusive boundaries and empty input.
Read the full Software Engineer interview experience this question came from
You are given an array of integers `scores`, where the index is a student ID and the value is that student's exam score as a percentage. Each student lands in exactly one grade band:
- a score **below 50** is **failed**;
- a score **from 50 to 80 inclusive** is **passed**;
- a score **above 80** is **passed with distinction**.
Return an array of exactly three integers `[failed, passed, distinction]`: the number of students who failed, the number who passed, and the number who passed with distinction, in that order.
### Rules
- A score of exactly 50 or exactly 80 counts as passed; a score of exactly 81 counts as passed with distinction.
- The three counts always sum to `len(scores)`.
- An empty array returns `[0, 0, 0]`.
### Constraints
- `0 <= len(scores) <= 100000`
- `0 <= scores[i] <= 100`
- Every count is at most 100000, so all values fit in a 32-bit signed integer.
### Example 1
- Input: `scores = [30, 50, 65, 80, 81, 95, 49]`
- Output: `[2, 3, 2]`
- Explanation: 30 and 49 fail; 50, 65 and 80 pass; 81 and 95 pass with distinction.
### Example 2
- Input: `scores = [100, 100]`
- Output: `[0, 0, 2]`
- Explanation: Both students scored above 80.
### Example 3
- Input: `scores = []`
- Output: `[0, 0, 0]`
- Explanation: With no students, every count is zero.
Constraints
- 0 <= len(scores) <= 100000
- 0 <= scores[i] <= 100
- Return exactly three counts [failed, passed, distinction]; each is at most 100000 and they sum to len(scores)
Examples
Input: ([30, 50, 65, 80, 81, 95, 49],)
Expected Output: [2, 3, 2]
Input: ([100, 100],)
Expected Output: [0, 0, 2]
Hints
- Every student belongs to exactly one band, so a single pass over the array with three counters is enough.
- Pin down the two cut points precisely: 49 and 50 fall on different sides, and so do 80 and 81.
- Check the lower band first, then the middle band's upper edge; whatever remains must be the top band.