Reverse singly linked list

Reverse every next link so the original tail becomes the new head.

Remember: Save next → reverse link → advance.

Example: 10 → 20 → 30 becomes 30 → 20 → 10.

Time: O(n). Auxiliary space: O(1).

Formula: next = current.next; current.next = previous

Each node keeps its identity even when its pointer changes. The input can contain repeated values.

Complexities describe the underlying algorithm; tutorial snapshots and rendering can use additional resources.

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