Leetcode #2776: Convert Callback Based Function to Promise Based Function
In this guide, we solve Leetcode #2776 Convert Callback Based Function to Promise Based Function in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Write a function that accepts another function fn and converts the callback-based function into a promise-based function. The function fn takes a callback as its first argument, along with any additional arguments args passed as separate inputs.
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: JavaScript
Intuition
We want to bridge a callback style into an awaitable one.
If we can capture the callback results into a future, we can await it just like a promise.
Approach
Create a wrapper coroutine that constructs a Future, passes a callback into fn, and resolves or rejects the future based on the callback.
Steps:
- Create a
Future. - Call
fn(callback, *args). - Resolve the future on success or set an exception on error.
awaitthe future and return its value.
Example
function sum(callback, a, b) {
if (a < 0 || b < 0) {
const err = Error('a and b must be positive');
callback(undefined, err);
} else {
callback(a + b);
}
}
Python Solution
import asyncio
def promisify(fn):
async def wrapped(*args):
loop = asyncio.get_running_loop()
future = loop.create_future()
def callback(value=None, err=None):
if future.done():
return
if err is not None:
future.set_exception(err)
else:
future.set_result(value)
fn(callback, *args)
return await future
return wrapped
Complexity
The time complexity is excluding the wrapped function, and the space complexity is .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.