Leet Code 41. First Missing Positive

https://leetcode.com/problems/first-missing-positive/ 

Example 1:

Input: nums = [3,4,-1,1]
Output: 2

jo bhi number inrange hai na, jese 5th index pr 2 mila toh 2 ko 2nd index wale number ke sath swap kr do, toh jo number present hain woh aopni sahi jgh pr pauch jaenge 

then, array ko traverse krnege toh, 0th index pr 1 hona chye, jh apr bhi aesa na ho, whi index wala number hi humara answer bnn jaega.

public class LeetCode_41_First_Missing_Positive {
    // brute force: pehle sort kr do, then, check kro pehle no. 1 hona cheye then 2...
    public static void main(String[] args) {
        int[] arr = new int[]{3, 4, -1, 1};
        System.out.print(firstMissingPositive(arr));
    }

    public static int firstMissingPositive(int[] nums) {
        int i = 0, n = nums.length;

        while (i < n) {
            if (nums[i] > 0 && nums[i] < n + 1 && nums[nums[i] - 1] != nums[i]) {
                swap(nums, i, nums[i] - 1);
            } else {
                i++;
            }
        }

        i = 0;
        while (i < n) {
            if (nums[i] != i++ + 1)
                return i;
        }

        return n + 1;
    }

    public static void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}