Solution
Approach 1: Schoolbook Addition
Intuition
Let's add numbers in a schoolbook way, column by column. For example, to add 123 and 912, we add 3+2, then 2+1, then 1+9. Whenever our addition result is more than 10, we carry the 1 into the next column. The result is 1035.
Algorithm
We can do a variant of the above idea that is easier to implement  we put the entire addend in the first column from the right.
Continuing the example of 123 + 912, we start with [1, 2, 3+912]. Then we perform the addition 3+912, leaving 915. The 5 stays as the digit, while we 'carry' 910 into the next column which becomes 91.
We repeat this process with [1, 2+91, 5]. We have 93, where 3 stays and 90 is carried over as 9. Again, we have [1+9, 3, 5] which transforms into [1, 0, 3, 5].
Complexity Analysis

Time Complexity: where is the length of
A
. 
Space Complexity: .
Analysis written by: @awice.