Why This O(n) Trick Beats Sorting Every Time (LeetCode 41)

In this video, we solve LeetCode Problem 41: First Missing Positive, one of the most important array problems asked in interviews.

The challenge is not the logic—it’s meeting the strict constraints:
✅ O(n) time
✅ O(1) extra space

Most people try sorting or hash sets and fail the constraints. Here, you’ll learn the index-as-a-hash trick that interviewers expect.

What you’ll learn:

Why sorting is disallowed (and why interviewers care)

How to use the array itself as a hash map

The exact placement logic for numbers 1 → n

How to detect the missing positive efficiently

Algorithm Used:

In-place hashing

Array index manipulation

Core Insight:

If a number x is in range [1, n], it belongs at index x - 1.

Time & Space Complexity:

Time: O(n)

Space: O(1)

Who this video is for:

Intermediate DSA learners

Interview preparation candidates

Anyone struggling with array constraints

Key Takeaway:

Constraints tell you how to think. Ignore them, and you fail interviews.

👍 Like, Subscribe, and Comment if you want more hard interview classics explained clearly.

Hashtags:

#LeetCode41 #FirstMissingPositive #Arrays #DSA #Python #CodingInterviewIn this video, we solve LeetCode Problem 41: First Missing Positive, one of the most important array problems asked in interviews.

The challenge is not the logic—it’s meeting the strict constraints:
✅ O(n) time
✅ O(1) extra space

Most people try sorting or hash sets and fail the constraints. Here, you’ll learn the index-as-a-hash trick that interviewers expect.

What you’ll learn:

Why sorting is disallowed (and why interviewers care)

How to use the array itself as a hash map

The exact placement logic for numbers 1 → n

How to detect the missing positive efficiently

Algorithm Used:

In-place hashing

Array index manipulation

Core Insight:

If a number x is in range [1, n], it belongs at index x - 1.

Time & Space Complexity:

Time: O(n)

Space: O(1)

Who this video is for:

Intermediate DSA learners

Interview preparation candidates

Anyone struggling with array constraints

Key Takeaway:

Constraints tell you how to think. Ignore them, and you fail interviews.

👍 Like, Subscribe, and Comment if you want more hard interview classics explained clearly.

Hashtags:

#LeetCode41 #FirstMissingPositive #Arrays #DSA #Python #CodingInterview