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You are given two **non-empty** linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

**Follow up:**

What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

**Example:**

Input:(7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)Output:7 -> 8 -> 0 -> 7

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