Fast & Slow Pointers

🟢 Easy

• 141. Linked List Cycle Easy Floyd's cycle detection – the absolute classic tortoise and hare problem.
• 876. Middle of the Linked List Easy Fast pointer reaches the end → slow pointer is perfectly at the middle.
• 202. Happy Number Easy Cycle detection applied to math (a sequence of digit‑square sums).
• 234. Palindrome Linked List Easy Find the middle with fast/slow pointers, then reverse the second half and compare.
• 160. Intersection of Two Linked Lists Easy Two pointers traversing at different offsets to neutralize length differences.
• 26. Remove Duplicates from Sorted Array Easy Array variation: Read pointer (fast) and Write pointer (slow) moving same direction.
• 283. Move Zeroes Easy Array variation: Slow pointer tracks placement of non-zeroes, fast pointer explores.
• 83. Remove Duplicates from Sorted List Easy Same‑direction pointer technique applied on a linked list.

🟠 Medium (that feel like the next step)

• 142. Linked List Cycle II Medium Find the exact node where the cycle begins. Direct mathematical follow‑up to #141.
• 287. Find the Duplicate Number Medium Array treated as a linked list with a cycle. A beautiful application of Cycle II logic.
• 19. Remove Nth Node From End of List Medium Fast pointer creates a gap of n nodes, then both move together to find the end.
• 2095. Delete the Middle Node of a Linked List Medium Uses fast/slow to find the middle node while keeping track of the previous node to delete it.
• 143. Reorder List Medium Find middle via fast/slow, reverse second half, then merge both halves together.
• 148. Sort List Medium Merge sort on a linked list – uses fast/slow pointer trick to cleanly split the list in half.
• 82. Remove Duplicates from Sorted List II Medium Delete all nodes that have duplicate numbers using a prev/curr (slow/fast) approach.
• 457. Circular Array Loop Medium Cycle detection applied to array indices with direction rules – a tricky twist on #141.

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* Difficulty tags follow LeetCode's official classification.