Fast and Slow Pointers

The "fast and slow pointers" pattern focuses on linked lists, specifically with linked lists that might have a cycle.

Problems for this Week

(easy) linked-list-cycle

Given head, the head of a linked list, determine if the linked list has a cycle in it. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

(medium) linked-list-cycle-ii

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter. Do not modify the linked list.

(easy) happy-number

Write an algorithm to return true if a number n is happy, or false otherwise. A happy number is a number defined by the following process:

  • Starting with any positive integer, replace the number by the sum of the squares of its digits.
  • Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
  • Those numbers for which this process ends in 1 are happy.

(easy) middle-of-the-linked-list

Given the head of a singly linked list, return the middle node of the linked list. If there are two middle nodes, return the second middle node.

(easy) palindrome-linked-list

Given the head of a singly linked list, return true if it is a palindrome or false otherwise.

(medium) reorder-list

You are given the head of a singly linked-list. The list can be represented as L0→L1→⋯→Ln−1→LnL_0 → L_1 → \dots → L_{n-1} → L_n.

Reorder the list to be of the form L0→Ln→L1→Ln−1→L2→Ln−2→…L_0 → L_n → L_1 → L_{n-1} → L_2 → L_{n-2} → \dots

You may not modify the values in the list's nodes. Only nodes themselves may be changed.

(medium) circular-array-loop

You are playing a game involving a circular array of non-zero integers nums. Each nums[i] denotes the number of indices forward/backward you must move if you are located at index i:

  • If nums[i] is positive, move nums[i] steps forward, and
  • If nums[i] is negative, move nums[i] steps backward.

Since the array is circular, you may assume that moving forward from the last element puts you on the first element, and moving backwards from the first element puts you on the last element.

A cycle in the array consists of a sequence of indices seq of length k where:

  • Following the movement rules above results in the repeating index sequence seq[0] -> seq[1] -> ... -> seq[k - 1] -> seq[0] -> ...
  • Every nums[seq[j]] is either all positive or all negative.
  • k > 1

Return true if there is a cycle in nums, or false otherwise.