Leetcode 41: First Missing Positive

Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

 

public class Solution {
    public int FirstMissingPositive(int[] nums) {
        for (int i = 0; i < nums.Length; i++)
        {
            while (nums[i] > 0 && nums[i] <= nums.Length && nums[nums[i] - 1] != nums[i])
            {
                var tmp = nums[nums[i] - 1];
                nums[nums[i] - 1] = nums[i];
                nums[i] = tmp;
            }
        }
        
        for (int i = 0; i < nums.Length; i++)
        {
            if (nums[i] != i + 1)
            {
                return i + 1;
            }
        }
        
        return nums.Length + 1;
    }
}