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In this post, you will find the solution for the **First Missing Positive** **in C++, Java & Python-LeetCode problem**. We are providing the **correct and tested solutions** to coding problems present on **LeetCode**. If you are not able to solve any problem, then you can take help from our Blog/website.

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** Link for the Problem** – First Missing Positive – LeetCode Problem

First Missing Positive– LeetCode Problem

**Problem:**

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 * 10`

^{5}`-2`

^{31}<= nums[i] <= 2^{31}- 1

First Missing Positive– LeetCode Solutions

class Solution { public: int firstMissingPositive(vector& nums) { const int n = nums.size(); // Correct slot: // nums[i] = i + 1 // nums[i] - 1 = i // nums[nums[i] - 1] = nums[i] for (int i = 0; i < n; ++i) while (nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i] - 1]) swap(nums[i], nums[nums[i] - 1]); for (int i = 0; i < n; ++i) if (nums[i] != i + 1) return i + 1; return n + 1; } };

class Solution { public int firstMissingPositive(int[] nums) { final int n = nums.length; // Correct slot: // nums[i] = i + 1 // nums[i] - 1 = i // nums[nums[i] - 1] = nums[i] for (int i = 0; i < n; ++i) while (nums[i] > 0 && nums[i] <= n && nums[i] != nums[nums[i] - 1]) swap(nums, i, nums[i] - 1); for (int i = 0; i < n; ++i) if (nums[i] != i + 1) return i + 1; return n + 1; } private void swap(int[] nums, int i, int j) { final int temp = nums[i]; nums[i] = nums[j]; nums[j] = temp; } }

class Solution: def firstMissingPositive(self, nums: List[int]) -> int: n = len(nums) # Correct slot: # nums[i] = i + 1 # nums[i] - 1 = i # nums[nums[i] - 1] = nums[i] for i in range(n): while nums[i] > 0 and nums[i] <= n and nums[nums[i] - 1] != nums[i]: nums[nums[i] - 1], nums[i] = nums[i], nums[nums[i] - 1] for i, num in enumerate(nums): if num != i + 1: return i + 1 return n + 1