# Coding Interviews

Given the head of a singly linked list, reverse the list, and return *the reversed list*.

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.

Merge the two lists into one **sorted** list. The list should be made by splicing together the nodes of the first two lists.

Return *the head of the merged linked list*.

Given the root of a binary tree, flatten the tree into a "linked list":

The "linked list" should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.

The "linked list" should be in the same order as a pre-order traversal of the binary tree.

Given the heads of two singly linked-lists headA and headB, return *the node at which the two lists intersect*. If the two linked lists have no intersection at all, return null.

Given the head of a linked list and a value x, partition it such that all nodes **less than** x come before nodes **greater than or equal** to x.

You should **preserve** the original relative order of the nodes in each of the two partitions.

Given an integer array nums where the elements are sorted in **ascending order**, convert *it to a ***height-balanced ***binary search tree*.

Given the root of a binary search tree and the lowest and highest boundaries as low and high, trim the tree so that all its elements lies in [low, high]. Trimming the tree should **not** change the relative structure of the elements that will remain in the tree (i.e., any node's descendant should remain a descendant). It can be proven that there is a **unique answer**.

Return *the root of the trimmed binary search tree*. Note that the root may change depending on the given bounds.

Given the root of a binary tree, return *the postorder traversal of its nodes' values*.

Given the root of a binary tree, invert the tree, and return *its root*.

Given the root of a binary tree, *determine if it is a valid binary search tree (BST)*.

A **valid BST** is defined as follows:

The left subtreeof a node contains only nodes with keys **less than** the node's key.

The right subtree of a node contains only nodes with keys **greater than** the node's key.

Both the left and right subtrees must also be binary search trees.

You are given the root of a binary search tree (BST), where the values of **exactly** two nodes of the tree were swapped by mistake. *Recover the tree without changing its structure*.

You are given two binary trees root1 and root2.

Imagine the trees are layed ontop of each other. The merged nodes will have the sum of the values of the original nodes. If one or the other is null, treat the null node as if it had a value of 0.

Return *the merged tree*.

Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return *the binary tree*.

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

The LCA is defined as: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we **allow a node to be a descendant of itself**).”

You are given a **0-indexed** array of **unique** strings words.

A **palindrome pair** is a pair of integers (i, j) such that:

0 <= i, j < words.length,

i != j, and

words[i] + words[j] (the concatenation of the two strings) is a palindrome.

Return *an array of all the ***palindrome pairs*** of *words.

You must write an algorithm with O(sum of words[i].length) runtime complexity.

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**.

Return true* if there is a cycle in the linked list*. Otherwise, return false.

Given a 1-indexed integer array prices, where prices[i] is the price of a particular stock on the ith day, your task is to select some of the elements of prices such that your selection is linear.

A selection indexes, where indexes is a 1-indexed integer array of length k which is a subsequence of the array [1, 2, ..., n], is linear if:

For every 1 < j <= k, prices[indexes[j]] - prices[indexes[j - 1]] == indexes[j] - indexes[j - 1]

The score of a linear selection is the sum of the prices at those indices, return the maximum score that a linear selection can have given the input.

You are given two **non-empty** linked lists representing two non-negative integers. The digits are stored in **reverse order**, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST.