Increment a number represented as a linked list. Each node contains a single digit and the most significant digit is stored in the head of the list.
Do not reverse the linked-list before performing the increment.
For example:
[ 1 ] --> [ 2 ] --> [ 3 ] => [ 1 ] --> [ 2 ] --> [ 4 ]
[ 1 ] --> [ 9 ] --> [ 9 ] => [ 2 ] --> [ 0 ] --> [ 0 ]
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