First Missing Positive

First Missing Positive is a Hard Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Array/Matrix Manipulation Patterns pattern, in the Array - Cyclic Sort subpattern.

Problem statement

Given an unsorted integer array nums. Return the smallest positive integer that is not present in nums.

You must implement an algorithm that runs in O(n) time and uses O(1) auxiliary space.

Example 1:

Input: nums = [1,2,0] Output: 3 Explanation: The numbers in the range [1,2] are all in the array.

Example 2:

Input: nums = [3,4,-1,1] Output: 2 Explanation: 1 is in the array but 2 is missing.

Example 3:

Input: nums = [7,8,9,11,12] Output: 1 Explanation: The smallest positive integer 1 is missing.

Constraints:

- 1 <= nums.length <= 105

- -231 <= nums[i] <= 231 - 1

Topics and companies

Topics: Array, Hash Table.

Reported in interviews at Microsoft, Amazon, Facebook, Google, Oracle, Adobe, Bloomberg, Wish, Uber, Apple, Walmart Labs, Tesla and others.

How to practise First Missing Positive on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n) time and O(1) space.

Open First Missing Positive in the code editor, or read the First Missing Positive editorial for a worked solution with its approach and complexity analysis.

Where First Missing Positive sits in the DSA pattern sheet

• Array/Matrix Manipulation Patterns pattern guide
• Array - Cyclic Sort subpattern
• The full DSA patterns sheet
• All coding practice problems

Problems related to First Missing Positive

Other problems that use the same Array - Cyclic Sort and Array/Matrix Manipulation Patterns techniques:

• Find All Duplicates in an Array — Medium, same subpattern
• Find All Numbers Disappeared in an Array — Easy, same subpattern
• Add Binary — Easy, same pattern
• Add to Array-Form of Integer — Easy, same pattern
• Diagonal Traverse — Medium, same pattern
• Game of Life — Medium, same pattern