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First Missing Integer Interviewbit Solution

Updated: May 29, 2021

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)