Write a program to reverse a singly linked list? Modify that program to reverse a doubly linked list.

Write a string to find a substring in a given string. Do this in O(n) or better

Write a function that takes an unsorted integer array, and returns a three element subset whose sum is zero.

Note: This is a special case of the SUBSET-SUM problem. That problem is NP-complete, but we're only looking for subsets of 3 elements. This can be achieved in polynomial time.

Write a function that takes as input a string, and outputs that same string, with any parentheses in it balanced. Do this using the minimum number of edits.

The least common multiple of a set of integers is the smallest positive integer that is a multiple of all of the integers in the set.

Write a function that takes an array of integers and efficiently calculates and returns the LCM.

Write function that takes a binary tree and efficiently returns the Nth smallest element.

For example, if N=4, and the tree looks like:

    3
   / \
  2   5
 /   / \
1   4   6

The function should return 4.

Write a function that takes as input a binary tree, and prints out each level of the tree on a newline. For example:

    a
   / \
  b   c
 /   / \
d   e   f

will output:

a
b c
d e f

Write a function that takes a an array of integers, and returns the contiguous subsequence of integers with the largest sum.