Fast and Slow Pointers

Determine if a given number is a "happy number," defined as a number where the sum of the squares of its digits eventually reaches 1, or remains at 1 in a loop, without descending to a negative value.

Given the head of a linked list, return the node where a cycle begins if there is a cycle, otherwise return null. Floyd's tortoise-and-hare algorithm finds the cycle and then the entry point in O(n) time with O(1) extra space.

Given an array nums containing n + 1 integers where each integer is in the inclusive range [1, n], there is exactly one duplicated value. Return the duplicated value without modifying nums and using only constant extra space.