Good morning! Here's our prompt for today.

We have an unsorted array of integers such as the following:

1[0, 3, -1, -2, 1]

In the above example, the minimum number is -2 and the maximum is 3. If it were sorted, it would look like:

1[-2, -1, 0, 1, 3]

This means there is an expected range (defined as the collection of values between the minimum and maximum values) of [-2, -1, 0, 1, 2, 3] for the array.

But look at the example again:

1[-2, -1, 0, 1, 3]

We're missing a number: 2. The smallest missing positive integer for our input array is 2.

Can you write a method that takes such an array and returns the first missing positive integer?

1leastMissingPositive([1, 2, 3, 0])
2// 4

In the above example, it is 4 since we already have 0 through 3. Have your code run in O(n) time with constant space (hence, we can easily determine if it was sorted, but most sorts will take O(n * log n) time).

Constraints

• Length of the array <= 100000
• The array can be empty
• The array will contain values between -100000 and 100000
• The final answer will always fit in the integer range
• Expected time complexity : O(n)
• Expected space complexity : O(1)

Try to solve this here or in Interactive Mode.

How do I practice this challenge?

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def least_missing_pos(nums):

    # fill in

    return nums

​

​

import unittest

​

​

class Test(unittest.TestCase):

    def test_1(self):

        assert least_missing_pos([3, 5, -1, 1]) == 2

        print("PASSED: Expect least_missing_pos([3,5,-1,1]) to return 2")

​

    def test_2(self):

        assert least_missing_pos([5, 6, 7, 8, 9]) == 1

        print("PASSED: Expect least_missing_pos([5,6,7,8,9]) to return 1")

​

    def test_3(self):

        assert least_missing_pos([]) == 1

        print("PASSED: Expect least_missing_pos([]) to return 1")

​

    def test_4(self):

        assert callable(least_missing_pos) == True

        print("PASSED: Expect least_missing_pos to be a function")

​

​

if __name__ == "__main__":

    unittest.main(verbosity=2)

    print("Nice job, 4/4 tests passed!")

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