[Leetcode] Replace Elements with Greatest Element on Right Side

Problem:

Given an array arr, replace every element in that array with the greatest element among the elements to its right, and replace the last element with -1.
After doing so, return the array.

Example:

Input: arr = [17,18,5,4,6,1]
Output: [18,6,6,6,1,-1]

Solution:

class Solution(object):
    def replaceElements(self, arr):
        """
        :type arr: List[int]
        :rtype: List[int]
        """
        
        arr.reverse()
        maximum_num = 0
        
        for i in range(len(arr)):
            if maximum_num < arr[i]:
                maximum_num = arr[i]
            if i == 0:
                maximum_num = arr[i]
                arr[i] = -1
            
            arr[i] = maximum_num
            
        for i in range(len(arr)-2, -1, -1):
            arr[i + 1] = arr[i]
        arr[0] = -1
        arr.reverse()
        return arr