Tagged Command Undo

Coding

Software Engineer

Create a command history that supports both ordinary undo and undo restricted to a particular tag.

Implement the CommandLog class:

• Calling CommandLog() creates a log with no commands.
• void execute(string command, string[] tags) records a command together with its tags. A command may be associated with no tags or with multiple tags.
• string undo(string tag) deletes and returns a single command from the log.
  • When tag is omitted or is null, select the newest command that remains active.
  • When a tag is supplied, select the newest active command containing that tag.

Additional rules:

• Commands follow execution order, with the latest execution being the newest command.
• A command may be undone at most one time.
• After a command has been undone, it must not be returned by any later call.

Choose a data structure that keeps every operation efficient across as many as 10^5 calls.

Examples

Example 1:

Input:
`["CommandLog","execute","execute","execute","undo","undo","undo"]
[[],["A",["x"]],["B",["y"]],["C",["x","y"]],[null],["x"],["y"]]`

Output: [null,null,null,null,"C","A","B"]

Explanation: The null undo takes the newest command overall, C. The subsequent x and y undos then select A and B, respectively.

Example 2:

Input:
`["CommandLog","execute","execute","execute","undo","undo","undo"]
[[],["alpha",[]],["beta",["prod"]],["gamma",[]],["prod"],[null],[null]]`

Output: [null,null,null,null,"beta","gamma","alpha"]

Constraints

• The combined number of execute and undo calls is at most 10^5.
• For every execute call, 0 <= tags.length <= 10.
• 1 <= command.length <= 100, and command is an opaque identifier.
• If a non-null tag is supplied, 1 <= tag.length <= 20.
• Every undo call is valid; a command available for that call always exists.

Starter Code

var CommandLog = function() {

};

/**
 * @param {string} command
 * @param {string[]} tags
 * @return {void}
 */
CommandLog.prototype.execute = function(command, tags) {

};

/**
 * @param {string | null} tag
 * @return {string}
 */
CommandLog.prototype.undo = function(tag) {

};