First Missing Positive
Given an unsorted integer array nums, find the smallest positive integer that does not appear in the array.
You must solve the problem in O(n) time and use O(1) extra space.
Example 1
Input
n = 3 nums = [1, 2, 0]
Output
3
Explanation
The positive integers start from 1.
1 and 2 are present, but 3 is missing.
Therefore, the answer is 3.
Example 2
Input
n = 4 nums = [3, 4, -1, 1]
Output
2
Explanation
1 is present, but 2 is missing.
Therefore, the smallest missing positive integer is 2.
Constraints
Hints:
Hint 1
The answer must be between 1 and n + 1.
Hint 2
Try placing every positive number x in the position x - 1.
Author & Technical Reviewer
Technically reviewed by: ExamAdda Technical Review Team
Technical Reviewers, ExamAdda
Software engineers at ExamAdda who check every article's definitions, complexity claims and code examples before and after publishing.
Published
Aug 27, 2026
Last updated
Aug 28, 2026
Content Verification Methodology
Definitions and complexity claims were checked against authoritative computer-science references. Code examples were compiled and tested with standard, boundary and edge-case inputs.
Expected Output