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You are given an array of `k` linked-lists `lists`, each linked-list is sorted in ascending order.

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

Given the head of a linked list, remove the nth node from the end of the list and return its head.

You are given the heads of two sorted linked lists list1 and list2.

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]] such that i !

Write a function to find the longest common prefix string amongst an array of strings.

Write a function, maxPathSum, that takes in the root of a binary tree that contains number values.