How should I practice Billing Log with Undo and Redo?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Reddit phone screen problems in FastPrep.

Is Billing Log with Undo and Redo associated with Reddit?

FastPrep tags this as a Reddit phone screen coding practice problem when the problem catalog has that company and stage context.

Problem · Array

Billing Log with Undo and Redo

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Problem statement

A billing amount starts at 0. Process paired arrays operations and amounts in order:

ADD adds the paired amount.
SET overwrites the current amount with the paired amount.
UNDO reverses the most recently applied ADD or SET, if one exists.
REDO reapplies the most recently undone change, if one exists.

A new ADD or SET after an undo discards the redo history. Amounts paired with UNDO and REDO are ignored. Return the current amount after every operation.

Function

billingStatusLog(operations: String[], amounts: long[]) → long[]

Examples

Example 1

operations = ["ADD","ADD","UNDO","REDO"]amounts = [5,3,0,0]return = [5,8,5,8]

Undo removes the second addition and redo reapplies it.

Example 2

operations = ["SET","ADD","UNDO","SET","REDO"]amounts = [10,5,0,7,0]return = [10,15,10,7,7]

The new SET after undo clears the redo history, so the final redo is a no-op.

Constraints

1 <= operations.length == amounts.length <= 100000
Every operation is ADD, SET, UNDO, or REDO.
-1000000000 <= amounts[i] <= 1000000000
Every intermediate amount fits in a signed 64-bit integer.