109. Convert Sorted List to Binary Search Tree

Problem

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

Solution

/**
 * 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) {
  return helper(head, null);
};

var helper = function (head, foot) {
  var fast = head;
  var slow = head;
  var node = null;

  if (head === foot) return null;

  while (fast !== foot && fast.next !== foot) {
    fast = fast.next.next;
    slow = slow.next;
  }

  node = new TreeNode(slow.val);
  node.left = helper(head, slow);
  node.right = helper(slow.next, foot);

  return node;
};

Explain:

nope.

Complexity:

• Time complexity : O(log(n)).
• Space complexity : O(n).