This problem was asked by Linkedin.
Given a sorted list of numbers, change it into a balanced binary search tree. You can assume there will be no duplicate numbers in the list.
Input: [1, 2, 3, 4, 5, 6, 7]
Output: "4261357"
# 4
# / \
# 2 6
# / \ / \
# 1 3 5 7