Given the head of a singly linked list where each node holds one digit of a non-negative number — most significant digit first — add 1 to the number and return the head of the resulting list. This looks like ordinary addition, but a linked list can only walk forward, while carrying a +1 naturally flows from the last digit toward the first. The optimal solution resolves that mismatch with a reversal trickReverse, Operate, Reverse BackA common pattern for list problems where the natural order of operation runs opposite to the list's natural direction of traversal: reverse the list so the operation becomes a simple forward pass, do the work, then reverse back to restore the original order — all in O(1) extra space. — reverse the list so the least significant digit comes first, add with a simple forward-carrying pass, then reverse back.