FirstMissingPositive.java

package com.saikat;

import java.util.ArrayList;
import java.util.List;

public class FirstMissingPositive {
    public static void main(String[] args) {
        int[] nums = {1,2,0};
        System.out.println(firstMissingPositive(nums));
    }
    static int firstMissingPositive(int[] nums) {
        int i = 0;
        while (i < nums.length){
            int correctIndex = nums[i] - 1;
            if (nums[i] > 0 && nums[i] <= nums.length && nums[i] != nums[correctIndex]){
                swap(nums,i,correctIndex);
            }else{
                i++;
            }
        }

        for(int index = 0; index < nums.length; index++){
            if(nums[index] != index + 1){
                return index + 1;
            }
        }
        return nums.length + 1;
    }
    static void swap(int[] arr, int first, int second){
        int temp = arr[first];
        arr[first] = arr[second];
        arr[second] = temp;
    }
}