Fast & Slow Pointers — Practice Problems

Ten problems that drill the fast & slow pointer pattern (Floyd’s tortoise-and-hare) from Lesson 5. The reps are the point — read the approach, try it yourself first, then check the code. Each solution here was actually run and tested.

How to practise. The trick with this pattern is always the same: a slow pointer moves one step, a fast pointer two. Before coding, say out loud what the meeting point tells you (a cycle exists, the middle node, the cycle’s entrance). Draw the list, trace both pointers by hand, then write it.

All the linked-list problems below assume this node definition:

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next