Given an array of integers, find the first missing positive integer in linear time and constant space

Question

In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well. This question was asked by Stri

Answer 1

Here's another python implementation with o(n) time complexity and o(1) space complexity

def segregate(arr):
    length = len(arr)
    neg_index = length
    for i, value in enumerate(arr):
        if(value < 1 and neg_index == length):
           neg_index = i
        if(neg_index != length and value >= 1):
           temp = arr[i]
           arr[i] = arr[neg_index]
           arr[neg_index] = temp
           neg_index += 1
    return arr[:neg_index]

def missingPositiveNumber(arr):
    arr = segregate(arr)
    length = len(arr)
    for i, value in enumerate(arr):
        if(value - 1 < l):
           arr[abs(value) - 1] = -(abs(arr[abs(value) - 1]))
    for i, value in enumerate(arr):
        if(value > 0):
           return i + 1
    return length + 1


print(missingPositiveNumber([1, -1, 2, 3]))