[vram22]

I've not tried to solve this question, but I think that by "mutate the inputs", tzs means mutate them as part of the algorithm you write, not before you use the inputs in your algorithm. E.g. it could be something like take a subset of the array, solve the problem for it, save that partial solution, then overwrite that part of the array for some reason, and repeat the same sort of logic for the other parts of the array, also using that saved partial solution (maybe) for the remaining work.

It's like the difference between sorting an array in-place (mutates the array as a whole (by changing order of items), but would not be legal sorting if it deleted or added any items), versus sorting it into a different array, leaving the original array intact. E.g. the difference between Python's lis.sort() and sorted(lis) where lis is a Python list.

[tzs]

Exactly. On LeetCode, and similar sites, you write some function whose prototype they supply. They evaluate your solution by calling your function with assorted inputs for which they know the correct answer.

Here's what they want you to write for the first missing positive problem if you are using Python3:

  class Solution:
      def firstMissingPositive(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """

If you are allowed to mutate the input, you can use nums.sort(). If you are not allowed to mutate input, you would need to do something like sortedNums = sorted(nums) if you needed a sorted version of nums.