Add to Array-Form of Integer

Solution

Schoolbook Addition

Add K to last element in A, do bitwise addition, then handle the carry.

Complexity

time: $O(max(n,\log K))$
space: $O(1)$

where n is length of A.

def addToArrayForm(self, A: List[int], K: int) -> List[int]:
    A[-1] += K
    for i in reversed(range(len(A))):
        carry, A[i] = divmod(A[i], 10)
        if i: A[i-1] += carry
    if carry:
        # OR list comprehension: A = [int(i) for i in str(carry)] + A
        A = list(map(int, str(carry))) + A
    return A