add solution to Convert Sorted List to Binary Search Tree
add solution to Flatten Binary Tree to Linked List
add solution to Construct Binary Tree from Preorder and Inorder Traversal
add solution to Construct Binary Tree from Inorder and Postorder Traversal
add solution to Merge Intervals
add solution to Insert Interval
add complexity
add solution to Recover Binary Search Tree
solution to Recover Binary Search Tree with constant space
simplify implementation

afterthought: minimize number of traversal
add solution to Gas Station
add solution to Count and Say
add solution to Jump Game II
add solution to Combination Sum