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Java-based LeetCode algorithm problem solutions, regularly updated
41\. First Missing Positive
Hard
Given an unsorted integer array `nums`, return the smallest missing positive integer.
You must implement an algorithm that runs in `O(n)` time and uses constant extra space.
**Example 1:**
**Input:** nums = [1,2,0]
**Output:** 3
**Example 2:**
**Input:** nums = [3,4,-1,1]
**Output:** 2
**Example 3:**
**Input:** nums = [7,8,9,11,12]
**Output:** 1
**Constraints:**
* 1 <= nums.length <= 5 * 105
* -231 <= nums[i] <= 231 - 1