LeetCode #2694 — MEDIUM

Event Emitter

Move from brute-force thinking to an efficient approach using core interview patterns strategy.

The Problem

Problem Statement

Design an EventEmitter class. This interface is similar (but with some differences) to the one found in Node.js or the Event Target interface of the DOM. The EventEmitter should allow for subscribing to events and emitting them.

Your EventEmitter class should have the following two methods:

subscribe - This method takes in two arguments: the name of an event as a string and a callback function. This callback function will later be called when the event is emitted.
An event should be able to have multiple listeners for the same event. When emitting an event with multiple callbacks, each should be called in the order in which they were subscribed. An array of results should be returned. You can assume no callbacks passed to subscribe are referentially identical.
The subscribe method should also return an object with an unsubscribe method that enables the user to unsubscribe. When it is called, the callback should be removed from the list of subscriptions and undefined should be returned.
emit - This method takes in two arguments: the name of an event as a string and an optional array of arguments that will be passed to the callback(s). If there are no callbacks subscribed to the given event, return an empty array. Otherwise, return an array of the results of all callback calls in the order they were subscribed.

Example 1:

Input: 
actions = ["EventEmitter", "emit", "subscribe", "subscribe", "emit"], 
values = [[], ["firstEvent"], ["firstEvent", "function cb1() { return 5; }"],  ["firstEvent", "function cb1() { return 6; }"], ["firstEvent"]]
Output: [[],["emitted",[]],["subscribed"],["subscribed"],["emitted",[5,6]]]
Explanation: 
const emitter = new EventEmitter();
emitter.emit("firstEvent"); // [], no callback are subscribed yet
emitter.subscribe("firstEvent", function cb1() { return 5; });
emitter.subscribe("firstEvent", function cb2() { return 6; });
emitter.emit("firstEvent"); // [5, 6], returns the output of cb1 and cb2

Example 2:

Input: 
actions = ["EventEmitter", "subscribe", "emit", "emit"], 
values = [[], ["firstEvent", "function cb1(...args) { return args.join(','); }"], ["firstEvent", [1,2,3]], ["firstEvent", [3,4,6]]]
Output: [[],["subscribed"],["emitted",["1,2,3"]],["emitted",["3,4,6"]]]
Explanation: Note that the emit method should be able to accept an OPTIONAL array of arguments.

const emitter = new EventEmitter();
emitter.subscribe("firstEvent, function cb1(...args) { return args.join(','); });
emitter.emit("firstEvent", [1, 2, 3]); // ["1,2,3"]
emitter.emit("firstEvent", [3, 4, 6]); // ["3,4,6"]

Example 3:

Input: 
actions = ["EventEmitter", "subscribe", "emit", "unsubscribe", "emit"], 
values = [[], ["firstEvent", "(...args) => args.join(',')"], ["firstEvent", [1,2,3]], [0], ["firstEvent", [4,5,6]]]
Output: [[],["subscribed"],["emitted",["1,2,3"]],["unsubscribed",0],["emitted",[]]]
Explanation:
const emitter = new EventEmitter();
const sub = emitter.subscribe("firstEvent", (...args) => args.join(','));
emitter.emit("firstEvent", [1, 2, 3]); // ["1,2,3"]
sub.unsubscribe(); // undefined
emitter.emit("firstEvent", [4, 5, 6]); // [], there are no subscriptions

Example 4:

Input: 
actions = ["EventEmitter", "subscribe", "subscribe", "unsubscribe", "emit"], 
values = [[], ["firstEvent", "x => x + 1"], ["firstEvent", "x => x + 2"], [0], ["firstEvent", [5]]]
Output: [[],["subscribed"],["subscribed"],["unsubscribed",0],["emitted",[7]]]
Explanation:
const emitter = new EventEmitter();
const sub1 = emitter.subscribe("firstEvent", x => x + 1);
const sub2 = emitter.subscribe("firstEvent", x => x + 2);
sub1.unsubscribe(); // undefined
emitter.emit("firstEvent", [5]); // [7]

Constraints:

1 <= actions.length <= 10
values.length === actions.length
All test cases are valid, e.g. you don't need to handle scenarios when unsubscribing from a non-existing subscription.
There are only 4 different actions: EventEmitter, emit, subscribe, and unsubscribe.
The EventEmitter action doesn't take any arguments.
The emit action takes between either 1 or 2 arguments. The first argument is the name of the event we want to emit, and the 2nd argument is passed to the callback functions.
The subscribe action takes 2 arguments, where the first one is the event name and the second is the callback function.
The unsubscribe action takes one argument, which is the 0-indexed order of the subscription made before.

Step 01

Brute Force Baseline

Problem summary: Design an EventEmitter class. This interface is similar (but with some differences) to the one found in Node.js or the Event Target interface of the DOM. The EventEmitter should allow for subscribing to events and emitting them. Your EventEmitter class should have the following two methods: subscribe - This method takes in two arguments: the name of an event as a string and a callback function. This callback function will later be called when the event is emitted. An event should be able to have multiple listeners for the same event. When emitting an event with multiple callbacks, each should be called in the order in which they were subscribed. An array of results should be returned. You can assume no callbacks passed to subscribe are referentially identical. The subscribe method should also return an object with an unsubscribe method that enables the user to unsubscribe. When it is

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: General problem-solving

Example 1

["EventEmitter", "emit", "subscribe", "subscribe", "emit"]
[[], ["firstEvent"], ["firstEvent", "function cb1() { return 5; }"],  ["firstEvent", "function cb1() { return 6; }"], ["firstEvent"]]

Example 2

["EventEmitter", "subscribe", "emit", "emit"]
[[], ["firstEvent", "function cb1(...args) { return args.join(','); }"], ["firstEvent", [1,2,3]], ["firstEvent", [3,4,6]]]

Example 3

["EventEmitter", "subscribe", "emit", "unsubscribe", "emit"]
[[], ["firstEvent", "(...args) => args.join(',')"], ["firstEvent", [1,2,3]], [0], ["firstEvent", [4,5,6]]]

Step 02

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

Source-backed implementations are provided below for direct study and interview prep.

// Accepted solution for LeetCode #2694: Event Emitter
// Auto-generated Java example from ts.
class Solution {
    public void exampleSolution() {
    }
}
// Reference (ts):
// // Accepted solution for LeetCode #2694: Event Emitter
// type Callback = (...args: any[]) => any;
// type Subscription = {
//     unsubscribe: () => void;
// };
// 
// class EventEmitter {
//     private d: Map<string, Set<Callback>> = new Map();
// 
//     subscribe(eventName: string, callback: Callback): Subscription {
//         this.d.set(eventName, (this.d.get(eventName) || new Set()).add(callback));
//         return {
//             unsubscribe: () => {
//                 this.d.get(eventName)?.delete(callback);
//             },
//         };
//     }
// 
//     emit(eventName: string, args: any[] = []): any {
//         const callbacks = this.d.get(eventName);
//         if (!callbacks) {
//             return [];
//         }
//         return [...callbacks].map(callback => callback(...args));
//     }
// }
// 
// /**
//  * const emitter = new EventEmitter();
//  *
//  * // Subscribe to the onClick event with onClickCallback
//  * function onClickCallback() { return 99 }
//  * const sub = emitter.subscribe('onClick', onClickCallback);
//  *
//  * emitter.emit('onClick'); // [99]
//  * sub.unsubscribe(); // undefined
//  * emitter.emit('onClick'); // []
//  */

// Accepted solution for LeetCode #2694: Event Emitter
// Auto-generated Go example from ts.
func exampleSolution() {
}
// Reference (ts):
// // Accepted solution for LeetCode #2694: Event Emitter
// type Callback = (...args: any[]) => any;
// type Subscription = {
//     unsubscribe: () => void;
// };
// 
// class EventEmitter {
//     private d: Map<string, Set<Callback>> = new Map();
// 
//     subscribe(eventName: string, callback: Callback): Subscription {
//         this.d.set(eventName, (this.d.get(eventName) || new Set()).add(callback));
//         return {
//             unsubscribe: () => {
//                 this.d.get(eventName)?.delete(callback);
//             },
//         };
//     }
// 
//     emit(eventName: string, args: any[] = []): any {
//         const callbacks = this.d.get(eventName);
//         if (!callbacks) {
//             return [];
//         }
//         return [...callbacks].map(callback => callback(...args));
//     }
// }
// 
// /**
//  * const emitter = new EventEmitter();
//  *
//  * // Subscribe to the onClick event with onClickCallback
//  * function onClickCallback() { return 99 }
//  * const sub = emitter.subscribe('onClick', onClickCallback);
//  *
//  * emitter.emit('onClick'); // [99]
//  * sub.unsubscribe(); // undefined
//  * emitter.emit('onClick'); // []
//  */

# Accepted solution for LeetCode #2694: Event Emitter
# Auto-generated Python example from ts.
def example_solution() -> None:
    return
# Reference (ts):
# // Accepted solution for LeetCode #2694: Event Emitter
# type Callback = (...args: any[]) => any;
# type Subscription = {
#     unsubscribe: () => void;
# };
# 
# class EventEmitter {
#     private d: Map<string, Set<Callback>> = new Map();
# 
#     subscribe(eventName: string, callback: Callback): Subscription {
#         this.d.set(eventName, (this.d.get(eventName) || new Set()).add(callback));
#         return {
#             unsubscribe: () => {
#                 this.d.get(eventName)?.delete(callback);
#             },
#         };
#     }
# 
#     emit(eventName: string, args: any[] = []): any {
#         const callbacks = this.d.get(eventName);
#         if (!callbacks) {
#             return [];
#         }
#         return [...callbacks].map(callback => callback(...args));
#     }
# }
# 
# /**
#  * const emitter = new EventEmitter();
#  *
#  * // Subscribe to the onClick event with onClickCallback
#  * function onClickCallback() { return 99 }
#  * const sub = emitter.subscribe('onClick', onClickCallback);
#  *
#  * emitter.emit('onClick'); // [99]
#  * sub.unsubscribe(); // undefined
#  * emitter.emit('onClick'); // []
#  */

// Accepted solution for LeetCode #2694: Event Emitter
// Rust example auto-generated from ts reference.
// Replace the signature and local types with the exact LeetCode harness for this problem.
impl Solution {
    pub fn rust_example() {
        // Port the logic from the reference block below.
    }
}

// Reference (ts):
// // Accepted solution for LeetCode #2694: Event Emitter
// type Callback = (...args: any[]) => any;
// type Subscription = {
//     unsubscribe: () => void;
// };
// 
// class EventEmitter {
//     private d: Map<string, Set<Callback>> = new Map();
// 
//     subscribe(eventName: string, callback: Callback): Subscription {
//         this.d.set(eventName, (this.d.get(eventName) || new Set()).add(callback));
//         return {
//             unsubscribe: () => {
//                 this.d.get(eventName)?.delete(callback);
//             },
//         };
//     }
// 
//     emit(eventName: string, args: any[] = []): any {
//         const callbacks = this.d.get(eventName);
//         if (!callbacks) {
//             return [];
//         }
//         return [...callbacks].map(callback => callback(...args));
//     }
// }
// 
// /**
//  * const emitter = new EventEmitter();
//  *
//  * // Subscribe to the onClick event with onClickCallback
//  * function onClickCallback() { return 99 }
//  * const sub = emitter.subscribe('onClick', onClickCallback);
//  *
//  * emitter.emit('onClick'); // [99]
//  * sub.unsubscribe(); // undefined
//  * emitter.emit('onClick'); // []
//  */

// Accepted solution for LeetCode #2694: Event Emitter
type Callback = (...args: any[]) => any;
type Subscription = {
    unsubscribe: () => void;
};

class EventEmitter {
    private d: Map<string, Set<Callback>> = new Map();

    subscribe(eventName: string, callback: Callback): Subscription {
        this.d.set(eventName, (this.d.get(eventName) || new Set()).add(callback));
        return {
            unsubscribe: () => {
                this.d.get(eventName)?.delete(callback);
            },
        };
    }

    emit(eventName: string, args: any[] = []): any {
        const callbacks = this.d.get(eventName);
        if (!callbacks) {
            return [];
        }
        return [...callbacks].map(callback => callback(...args));
    }
}

/**
 * const emitter = new EventEmitter();
 *
 * // Subscribe to the onClick event with onClickCallback
 * function onClickCallback() { return 99 }
 * const sub = emitter.subscribe('onClick', onClickCallback);
 *
 * emitter.emit('onClick'); // [99]
 * sub.unsubscribe(); // undefined
 * emitter.emit('onClick'); // []
 */

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n)

Space

O(1)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.