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g0001_0100.s0041_first_missing_positive.readme.md Maven / Gradle / Ivy

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

**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