Reverse Linked List

Statement

Given the head of a singly linked list, reverse the list in place and return the new head.

Examples

head = 1 → 2 → 3 → 4 → 5 → null
→ 5 → 4 → 3 → 2 → 1 → null

head = 1 → 2 → null
→ 2 → 1 → null

head = null
→ null

head = 1 → null
→ 1 → null

Constraints

• 0 ≤ list length ≤ 5000
• -5000 ≤ Node.val ≤ 5000
• Solve in O(1) extra memory if asked.

What this problem is really testing

The simplest possible exam of pointer discipline: do you understand that assigning curr.next = prev destroys the only reference to the original successor unless you save it first? Every linked-list problem you'll see later (cycle detection, merging, reordering) relies on this exact pattern. It's the four-line drill that earns its place by being the substrate of everything that follows.