Event Loop Simulator - Problem

Implement a single-threaded event loop simulator that processes tasks from multiple queues with proper priority handling.

Your simulator must handle:

Task Queue: Regular tasks (like DOM events, I/O callbacks)
Microtask Queue: High-priority tasks (like Promise.then(), queueMicrotask())
Timer Queue: Delayed tasks (like setTimeout, setInterval)

The event loop follows JavaScript's execution model:

Execute all available microtasks first
Execute one task from the task queue
Execute all microtasks again
Check for timer tasks that are ready to run
Repeat until all queues are empty

Given a list of operations with timestamps, return the execution order of all tasks.

Input & Output

Example 1 — Basic Event Loop

$ Input: operations = [[0, "task", "A"], [0, "microtask", "B"], [5, "timer", "C", 5]]

› Output: ["B", "A", "C"]

💡 Note: At time 0: microtask B executes first (highest priority), then task A. At time 10: timer C is ready and executes.

Example 2 — Multiple Microtasks

$ Input: operations = [[0, "task", "X"], [0, "microtask", "Y"], [0, "microtask", "Z"]]

› Output: ["Y", "Z", "X"]

💡 Note: All microtasks (Y, Z) execute first in order, then task X executes.

Example 3 — Timer Scheduling

$ Input: operations = [[0, "timer", "P", 0], [5, "task", "Q"]]

› Output: ["P", "Q"]

💡 Note: Timer P with 0 delay executes immediately at time 0, task Q executes at time 5.

Constraints

1 ≤ operations.length ≤ 1000
0 ≤ timestamp ≤ 1000
0 ≤ delay ≤ 100
Each task ID is unique

Visualization

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Asked in

G Google 35 f Meta 28 M Microsoft 22 a Amazon 18

The key insight is implementing JavaScript's event loop semantics with three priority queues. Microtasks always execute first, followed by one task, then checking timers. Best approach uses separate queues with proper priority handling. Time: O(n log n), Space: O(n)

Common Approaches

✓ Event Loop with Three Queues

⏱️ Time: O(n log n) Space: O(n)

Use three separate queues and follow JavaScript's event loop semantics: microtasks have highest priority, followed by tasks, then timers when ready.

Simple Queue Processing

⏱️ Time: O(n log n) Space: O(n)

Execute operations as they arrive in time order, ignoring the event loop's priority system. This doesn't follow JavaScript's actual execution model.

Event Loop with Three Queues — Algorithm Steps

Parse operations into three queues by type
Process all microtasks first
Execute one task, then check microtasks again
Handle ready timers and repeat cycle

Visualization

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Step-by-Step Walkthrough

Separate Queues

Tasks sorted into three priority queues

Process Microtasks

Handle all high-priority tasks first

Event Loop Cycle

One task, then microtasks, repeat

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Simplified implementation for C
char** solution(char operations[][100], int n, int* returnSize) {
    char** result = malloc(n * sizeof(char*));
    *returnSize = 0;
    
    // Simple event loop simulation
    for (int i = 0; i < n; i++) {
        char* taskId = malloc(20);
        sprintf(taskId, "T%d", i + 1);
        result[(*returnSize)++] = taskId;
    }
    
    return result;
}

int main() {
    // Simplified input parsing
    int n = 3;
    char operations[10][100];
    int returnSize;
    
    char** result = solution(operations, n, &returnSize);
    
    printf("[");
    for (int i = 0; i < returnSize; i++) {
        printf("\"%s\"", result[i]);
        if (i < returnSize - 1) printf(",");
    }
    printf("]\n");
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n log n)

Sorting operations and managing priority queues

⚡ Linearithmic

Space Complexity

O(n)

Three separate queues plus execution results

⚡ Linearithmic Space

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Event Loop Simulator - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Event Loop with Three Queues — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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