DSA TrainerFind All Numbers Disappeared in an Array
Given an array of n integers where each integer is in the range [1, n], return all integers in [1, n] that do not appear in the array. Some numbers may appear twice.
This is a premium problem.
Unlock Find All Numbers Disappeared in an Array with the full guided learning path and every other premium problem.
Problem
Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.
Input
An integer array `nums` of length n where each element is in [1, n].
Output
A list of all integers from 1 to n that are missing from `nums`.
Examples
Input: nums = [4,3,2,7,8,2,3,1]
Output: [5,6]
Input: nums = [1,1]
Output: [2]
The brute-force approach
Use a hash set of all numbers in nums. Scan 1 to n, collect those not in the set.
seen = set(nums)
return [i for i in range(1, n+1) if i not in seen]O(n) time but O(n) extra space. Cyclic sort achieves O(n) time and O(1) extra space.
Spotting the pattern
This is a Cyclic Sort problem. The key question to ask yourself:
After the cyclic sort, why does nums[i] != i+1 at some index tell you exactly which number is missing? What should nums[i] equal if the array were complete?
Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.
Unlock the full guided solution
The Cyclic Sort walkthrough for Find All Numbers Disappeared in an Array: the progressive hint ladder, a row-by-row dry run, the optimized code, and an in-browser runner, plus the full guided learning path and every other premium problem.
Already finished Big-O and Hash Maps free? This picks up right where the path leads: same format, harder patterns, the ones that show up in every FAANG-style screen.
What you get
- ✓The full guided learning path: concept lessons, drills, graded tests, and cold-read capstones for every pattern
- ✓Stop going blank on algorithm problems. The hint ladder walks you to the answer without spoiling it.
- ✓Actually internalize the trace: dry runs walk state row by row so it clicks before you write a line
- ✓Know when to reach for each pattern. Every problem names the trigger so you spot it next time.
- ✓Write and run JS or Python in the browser with real test cases on every problem
- ✓Access every new problem, unit, and pattern guide as they ship weekly
“This app finally made it click. I tried NeetCode, CTCI, and YouTube for years and still couldn't solve Two Sum. The beginner mental models for the patterns are genuinely so helpful. THANK YOU.”
Founding Member Deal
$29 / year
or $6 / month
LeetCode Premium is $159/year and doesn't teach you anything.
Less than one Udemy course, with guided problems instead of passive lectures.
One extra month of job searching costs more than a full year here.
Your price is locked in for life.
When the 100th founding spot closes, the price goes up for everyone who joins after. Your rate never changes, no matter how much the curriculum grows.
- ✓The full guided learning path, every unit
- ✓All 150 problems across 33 patterns, plus every new one
- ✓New problems and units weekly
- ✓Cancel anytime
Cancel anytime. Not useful within 7 days? Email for a full refund. Secured by Stripe.