class Solution:
  def findMissingNumber(self, nums):
    
    def swap(i, j):
       nums[i], nums[j] = nums[j], nums[i]
    
    # try to position each element such as nums[i] == i. 
    # the max element can not placed as nums[i] == i.
    # e.g. [4, 0, 3, 1] max = 4 
    # as result of placing all the elements you will have the max in the missing index i. 
    for i in range(len(nums)):
      while nums[i] != i and nums[i] != len(nums):
        swap(i, nums[i])

    for i in range(len(nums)):
      if nums[i] == len(nums):
        return i

    return len(nums)