Linked Lists & Pointers

Linked lists carry more weight in a PayPal loop than at most shops, which is why they get their own lesson here. These recur across PayPal coding rounds at the easy-to-medium tier: reverse a list, detect or locate a cycle, merge two sorted lists, reverse in groups of k. Adding two numbers stored as digits and building an O(1) structure on top of a list round out the set. The problems are rarely exotic. An interviewer is watching whether your pointer code is clean, your rewiring order is correct, and your edge cases hold.

Three moves carry almost everything here. A dummy head removes the special case where the front of the answer changes. Fast and slow pointers expose a midpoint, a cycle, or the kth node from the end in a single pass. In-place reversal flips each link while marching forward with three pointers. None of these is a trick. Each is a small skeleton you can write from memory, narrate out loud, and stack when a problem needs two of them at once.