One-hot encode an integer array

Write one_hot(arr: list[int]) -> list[list[int]] that returns the one-hot encoding of an integer array as a 2D list.

• The matrix has one row per element of arr, in the same order.
• It has one column per distinct value in arr. Columns are ordered by value, smallest first.
• Row i has a 1 in the column for arr[i] and 0 everywhere else.

An empty input returns an empty list.

Constraints

• 0 <= len(arr) <= 10^5
• -10^9 <= arr[i] <= 10^9
• The number of distinct values times len(arr) fits in memory.

Example 1

arr = [4, 1, 4, 7]
columns (sorted distinct values): [1, 4, 7]
output:
[[0, 1, 0],
 [1, 0, 0],
 [0, 1, 0],
 [0, 0, 1]]

Example 2

arr = [3, 3, -2]
columns: [-2, 3]
output: [[0, 1], [0, 1], [1, 0]]

Possible next parts: decode a matrix back to the values, return a compact representation when there are many distinct values, and encode new data with columns fixed from earlier data.

Reconstructed from the reported question; details may differ from what interviewers ask.