A number chain is a sequence of at least three non-negative integers where every number from the
third on equals the sum of the two before it. A clerk wrote a chain as one digit string digits,
without separators and without leading zeros (the number 0 itself is written "0"). Return True
if digits can be split into such a chain, otherwise False.
Examples
Input: digits = "358"
Output: True
Explanation: 3, 5, 8.
Input: digits = "1023"
Output: False
Input: digits = "000"
Output: True
Explanation: 0, 0, 0.
Constraints
1 <= len(digits) <= 30, digits only
Goals
- Recognise that the first two choices determine the whole sequence
- Validate the rest of the string greedily after the choices