Linked Lists¶

Key Patterns¶

1. Two Pointers (Fast & Slow)¶

When: Cycle detection, finding middle, nth from end
How: Fast moves 2x, slow moves 1x

2. Dummy Node¶

When: Operations that might change head
How: Create dummy pointing to head, return dummy.next

3. Reversal¶

When: Reverse entire list or portion
How: Track prev, curr, next pointers

4. Merge¶

When: Combining sorted lists
How: Compare heads, build result

Problems by Pattern¶

Fast & Slow Pointers¶

Problem	Difficulty	Link	Status
Linked List Cycle	Easy
Middle of Linked List	Easy
Remove Nth Node From End	Medium

Reversal¶

Problem	Difficulty	Link	Status
Reverse Linked List	Easy
Reverse Linked List II	Medium
Reverse Nodes in k-Group	Hard

Merge¶

Problem	Difficulty	Link	Status
Merge Two Sorted Lists	Easy
Merge k Sorted Lists	Hard

Common Techniques¶

# Fast & Slow (cycle detection)
slow = fast = head
while fast and fast.next:
    slow = slow.next
    fast = fast.next.next
    if slow == fast:
        return True  # cycle exists

# Dummy node
dummy = ListNode(0)
dummy.next = head
# ... operations
return dummy.next

# Reverse linked list
prev, curr = None, head
while curr:
    next_temp = curr.next
    curr.next = prev
    prev = curr
    curr = next_temp
return prev

Edge Cases¶

Empty list (head is None)
Single node
Two nodes
Cycle at different positions