Algorithm: zeroes on the right, non-zeroes on left

zeroes.py

# Given an list of random numbers, push all the zeroes to the end of the list. 
# For example, if the given list is [1, 9, 8, 4, 0, 0, 2, 7, 0, 6, 0], 
# it should be changed to [1, 9, 8, 4, 2, 7, 6, 0, 0, 0, 0].
# Time complexity should be O(n).

def order(arr):
    idx = 0
    ldx = len(arr) - 1
    while idx < ldx:
        a = arr[idx]
        b = arr[ldx]
        if not a:
            if b:
                arr[ldx] = a
                arr[idx] = b
                idx += 1
            ldx -= 1
        elif b:
            if not a:
                arr[ldx] = a
                arr[idx] = b
                ldx -= 1
            idx += 1
        else:
            idx += 1
            ldx -= 1