First Missing Integer Interviewbit Solution

Problem: First Missing Integer

Problem Description:

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

Example:

Given [1,2,0] return 3,
[3,4,-1,1] return 2,
[-8, -7, -6] returns 1

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

Solution:

Method 1:

Time Complexity: O(N) 
Space Complexity: O(N)

Method 2:

Time Complexity: O(NlogN)
Space Complexity: O(1)

Method 3:

Time Complexity: O(N)
Space Complexity: O(1)