Add Two Numbers as Lists

medium

A big-integer library stores numbers as linked lists of decimal digits in reverse order — least significant digit first — because that's the order addition consumes them. 342 is stored as 2 → 4 → 3.

Given two non-negative integers in this format, return their sum as a new list in the same format. The numbers can be far larger than any built-in integer width is meant to flex — work digit by digit with a carry. Implement _add(a, b) on real nodes.

Example 1

in a = [2, 4, 3], b = [5, 6, 4]

out [7, 0, 8]

342 + 465 = 807, stored reversed.

Example 2

in a = [0], b = [0]

out [0]

Example 3

in a = [9, 9, 9, 9, 9, 9, 9], b = [9, 9, 9, 9]

out [8, 9, 9, 9, 0, 0, 0, 1]

9,999,999 + 9,999 = 10,009,998 — the carry ripples and extends the result.

Constraints

· 1 <= nodes in each list <= 100
· 0 <= digit <= 9
· no leading zeros except the number 0 itself