# [41. First Missing Positive](https://leetcode.com/problems/first-missing-positive/)
###### tags: `Array`, `Bloomberg`

Description:

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

Solution:

class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            while (nums[i] > 0 && nums[i] <= n && nums[nums[i] - 1] != nums[i]) {
                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) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}