first-positive.py

class Solution:
    def firstMissingPositive(self, nums):
        nums += [0]
        for i, num in enumerate(nums):
            while nums[i] > 0 and nums[i] < len(nums) and nums[i] != i:
                j = nums[i]
                prev = nums[i]
                nums[i], nums[j] = nums[j], nums[i]
                if nums[i] == prev: break
                
        for i in range(1, len(nums)):
            if nums[i] != i: return i
        return len(nums)