<p>You have two numbers represented by a linked list, where each node contains a single digit. The digits are stored in reverse order, such that the 1's digit is at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list.</p>
<p> </p>
<p><strong>Example: </strong></p>
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<strong>Input: </strong>(7 -> 1 -> 6) + (5 -> 9 -> 2). That is, 617 + 295.
<strong>Output: </strong>2 -> 1 -> 9. That is, 912.
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<p><strong>Follow Up: </strong>Suppose the digits are stored in forward order. Repeat the above problem.</p>
<p><strong>Example: </strong></p>
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<strong>Input: </strong>(6 -> 1 -> 7) + (2 -> 9 -> 5). That is, 617 + 295.
<strong>Output: </strong>9 -> 1 -> 2. That is, 912.
