Recognition Signals (When to use this pattern)
- →Reverse a linked list or part of it
- →Detect or find the start of a cycle
- →Find the middle node or kth from the end
- →Merge two or more sorted lists
- →Remove or reorder nodes in-place
- →Clone a list with random / next pointers
1
Types of Problems (Subpatterns)
Each subpattern solves a specific type of problem. Click on a card to view its template.
2
Template / Approach
Template for: Reversal
1
Track Three Pointers
Keep prev = nullptr, curr = head, and next = nullptr before the loop begins.
2
Save Next
Store next = curr->next before overwriting the pointer — otherwise you lose the rest of the list.
3
Flip and Advance
Set curr->next = prev, then advance: prev = curr, curr = next. Repeat until curr is nullptr.
4
Return New Head
When the loop ends, prev points to the new head of the reversed list.
template_cpp
💡 Universal iterative reversal skeleton. Works for full reversal, and is a building block for partial reversal (reverse between positions) and k-group reversal.
1// Iterative reversal — O(N) time, O(1) space2ListNode* reverseList(ListNode* head)3{4 ListNode* prev = nullptr;5 ListNode* curr = head;67 while (curr)8 {9 ListNode* next = curr->next; // save before overwriting1011 curr->next = prev; // flip pointer12 prev = curr; // advance prev13 curr = next; // advance curr14 }1516 return prev; // new head17}
Key Idea:Reversal requires exactly three pointers: prev (already reversed), curr (being processed), next (saved before overwrite). The direction of curr->next flips each iteration — no extra space needed.
Practice Problems
Sharpen your skills with hand-picked problems.
Easy
Reverse Linked List
LeetCode #206
Medium
Reverse Linked List II
LeetCode #92
Hard
Reverse Nodes in k-Group
LeetCode #25
Medium
Swap Nodes in Pairs
LeetCode #24
Easy
Linked List Cycle
LeetCode #141
Medium
Linked List Cycle II
LeetCode #142
Easy
Middle of the Linked List
LeetCode #876
Medium
Reorder List
LeetCode #143
Easy
Palindrome Linked List
LeetCode #234
Medium
Remove Nth Node From End of List
LeetCode #19
Easy
Merge Two Sorted Lists
LeetCode #21
Medium
Remove Duplicates from Sorted List II
LeetCode #82
Medium
Partition List
LeetCode #86
Hard
Merge K Sorted Lists
LeetCode #23
Medium
Sort List
LeetCode #148
Medium
Insertion Sort List
LeetCode #147
Medium
Odd Even Linked List
LeetCode #328
Medium
Rotate List
LeetCode #61
Medium
Copy List with Random Pointer
LeetCode #138
Medium
Delete Node in a Linked List
LeetCode #237
Easy
Intersection of Two Linked Lists
LeetCode #160
Medium
Find the Duplicate Number
LeetCode #287
Medium
LRU Cache
LeetCode #146