Last updated on October 10th, 2024 at 02:11 am

Here, We see First Missing Positive LeetCode Solution. This Leetcode problem is done in many programming languages like C++, Java, JavaScript, Python, etc., with different approaches.

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First Missing Positive LeetCode Solution

Problem Statement

Given an unsorted integer array nums, return the smallest missing positive integer.

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.

1. First Missing Positive Leetcode Solution C++

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        int n=nums.size();
        for(int i=0 ; i<n ; i++)
        {
            while(nums[i]>0 and nums[i]<=n and 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;      
    }
};

2. First Missing Positive Leetcode Solution Java

class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        boolean[] exists = new boolean[n];
        for (int num : nums) {
            if (num > 0 && num <= n) exists[num - 1] = true;
        }
        for (int i = 0; i < exists.length; i++) {
            if (!exists[i]) return i + 1;
        }
        return n + 1;       
    }
}

3. First Missing Positive Leetcode Solution JavaScript

var firstMissingPositive = function(nums) {
    for (let i = 0; i < nums.length; i++) {
        let idx = nums[i]-1;
        if (i == idx || nums[i] == nums[idx]) continue; 
        if (idx >= 0 && idx <= nums.length - 1) {
            [nums[i], nums[idx]] = [nums[idx], nums[i]];
            i--; 
        }
    }
    for (let i = 0; i < nums.length; i++) {
        if (i+1 == nums[i]) continue;
        else return i+1; 
    }
    return nums.length + 1; 
};

4. First Missing Positive Leetcode Solution Python

class Solution(object):
    def firstMissingPositive(self, nums):
        unique = set(nums)
        i = 1
        while i in unique:
            i += 1
        return i