How should I practice Shell Command Counts with History References?

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Shell Command Counts with History References

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Problem statement

Process the entries of history in order. Each entry is a base command cp, ls, or mv, or a history reference !index.

A reference executes the same base command that was executed by the entry at the one-based position index. That earlier entry may itself be a reference. Each history entry executes exactly one base command.

Return the total execution counts in the order [cp, ls, mv], including direct commands and references.

For this exercise, assume every reference points to an earlier entry and uses a positive decimal index without leading zeros. All input entries are valid.

Function

countShellCommands(history: String[]) → int[]

Examples

Example 1

history = ["cp","ls","!1","mv","!3"]return = [3,1,1]

The executed commands are cp, ls, cp, mv, and cp. The last entry references an earlier reference.

Example 2

history = ["mv","!1","!2","ls"]return = [0,1,3]

The first three entries all execute mv; the fourth executes ls.

Constraints

For this exercise, assume 1 <= history.length <= 10^5.
Each entry is cp, ls, mv, or !index.
For a reference at one-based position i, 1 <= index < i.