An additive number is a string whose digits can form an additive sequence.

A valid additive sequence should contain at least three numbers. Except for the first two numbers, each subsequent number in the sequence must be the sum of the preceding two.

Given a string containing only digits, return `true` if it is an additive number or `false` otherwise.

Note: Numbers in the additive sequence cannot have leading zeros, so sequence `1, 2, 03` or `1, 02, 3` is invalid.

Example 1:

    Input: "112358"
    Output: true
    Explanation:
    The digits can form an additive sequence: 1, 1, 2, 3, 5, 8.
    1 + 1 = 2, 1 + 2 = 3, 2 + 3 = 5, 3 + 5 = 8

Example 2:

    Input: "199100199"
    Output: true
    Explanation:
    The additive sequence is: 1, 99, 100, 199.
    1 + 99 = 100, 99 + 100 = 199

Constraints:

- `1 <= num.length <= 35`
- `num` consists only of digits.

Follow up: How would you handle overflow for very large input integers?