Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number[]} nums
 * @return {TreeNode}
 */
var sortedArrayToBST = function(nums) {
  return dfs(0, nums.length - 1);

  function dfs(start, end) {
    if (start > end)
      return null;

    var mid = (start + end) >> 1;
    var node = new TreeNode(nums[mid]);
    node.left = dfs(start, mid - 1);
    node.right = dfs(mid + 1, end);

    return node;
  }
};

Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {number[]} nums
 * @return {TreeNode}
 */
var sortedArrayToBST = function(nums) {
  return dfs(0, nums.length - 1);

  function dfs(start, end) {
    if (start > end)
      return null;

    var mid = (start + end) >> 1;
    var node = new TreeNode(nums[mid]);
    node.left = dfs(start, mid - 1);
    node.right = dfs(mid + 1, end);

    return node;
  }
};


/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {TreeNode}
 */
var sortedListToBST = function(head) {
  var ans = [];
  while (head) {
    ans.push(head.val);
    head = head.next;
  }

  return sortedArrayToBST(ans);
};

Copy List with Random Pointer

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.

/**
 * Definition for singly-linked list with a random pointer.
 * function RandomListNode(label) {
 *     this.label = label;
 *     this.next = this.random = null;
 * }
 */

/**
 * @param {RandomListNode} head
 * @return {RandomListNode}
 */
var copyRandomList = function(head) {
  if (!head)
    return null;

  let hash = new Map();
  let newArr = [];
  let node = head;

  while (node) {
    hash.set(node, newArr.length);
    newArr.push(new RandomListNode(node.label));
    node = node.next;
  }

  node = head;
  for (let i = 0, len = newArr.length; i < len; i++) {
    if (i !== len - 1)
      newArr[i].next = newArr[i + 1];

    let random = node.random;
    let index = hash.get(random);
    if (index !== undefined) {
      newArr[i].random = newArr[index];
    }

    node = node.next;
  }

  return newArr[0];
};