Letter Grades in Swift

Given an array of integers scores, return the letter grade of every score, in the same order as the scores.

Each score gets one letter:

• 90 to 100: "A"
• 80 to 89: "B"
• 70 to 79: "C"
• 60 to 69: "D"
• 0 to 59: "F"

The work is split between two functions. The first one is a helper: it gets one score and returns its letter. The second one, the function that is graded, goes over the scores and collects the helper's letter for each of them.

Example 1:

Input: scores = [95, 82, 71]
Output: ["A", "B", "C"]
Explanation: 95 is in the A range (90 to 100), 82 is in the B range and 71 is in the C range.

Example 2:

Input: scores = [65, 40]
Output: ["D", "F"]
Explanation: 65 is between 60 and 69, so it gets a D. 40 is below 60, so it gets an F.

Example 3:

Input: scores = [100, 90, 89]
Output: ["A", "A", "B"]
Explanation: 100 and 90 are both in the A range: 90 is its lowest score. 89 is one point short of it, so it gets a B.

Constraints:

• scores has at least one score.
• Every score is an integer from 0 to 100.
• The result has one letter per score, in the same order as scores.