Fast, Slow pointer

• 141. Linked List Cycle
• 142. Linked List Cycle II
• 202. Happy Number
• 876. Middle of the Linked List: Very easy, when the fast pointer reaches the end, the slow pointer is at the middle.
• 234. Palindrome Linked List: Get to the middle and then reverse the next half, then check that both halfs are the same.
• 143. Reorder List

Linked List Cycle and Cycle start explained

If we have one fast and one slow pointer, if they meet there is a cycle.

Happy Number