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Reverse a Linked List

In-Place Pointer Reversal — Three-Pointer Technique

LeetCode 206 • Easy • Linked Lists

Input: [1, 2, 3, 4, 5] → Output: [5, 4, 3, 2, 1]

Time O(n) traverse list once

Space O(1) only 3 pointers

Reversed: 0/5 Pointers: prev, curr, next

current

next_temp

Reversed link

Original link

Speed:

Ready

Press Play to watch the reversal, or Step to advance one sub-step at a time.
Three pointers (prev, current, next_temp) walk through the list, flipping each arrow as they go.

✎ Whiteboard

Size: 3

⌨ Type It — Build Muscle Memory

Practice until you don't need to look. Use the guide comments below as scaffolding. Type the full implementation in the editor. The green highlights are the nuances to burn into memory.

# ─── REVERSE LINKED LIST (LeetCode 206) ───

# Pattern: Three-pointer in-place reversal

# Time: O(n) Space: O(1)

# 1. Define: reverseList(self, head)

# 2. Init two pointers:

# prev = None ← nothing before head

# current = head ← start at first node

# 3. Loop: while current:

# 4. Inside loop — 4 lines every time:

# next_temp = current.next ← save before breaking

# current.next = prev ← reverse the link!

# prev = current ← advance prev

# current = next_temp ← advance current

# 5. Return: return prev ← new head

# Vars: prev, current, next_temp

# Data structure: ListNode with .next pointer

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