ID	Title	Difficulty
94	Binary Tree Inorder Traversal	Medium
95	Unique Binary Search Trees II	Medium
96	Unique Binary Search Trees	Medium
98	Validate Binary Search Tree	Medium
99	Recover Binary Search Tree	Hard
100	Same Tree	Easy
101	Symmetric Tree	Easy
102	Binary Tree Level Order Traversal	Medium
103	Binary Tree Zigzag Level Order Traversal	Medium
104	Maximum Depth of Binary Tree	Easy
105	Construct Binary Tree from Preorder and Inorder Traversal	Medium
106	Construct Binary Tree from Inorder and Postorder Traversal	Medium
107	Binary Tree Level Order Traversal II	Easy
108	Convert Sorted Array to Binary Search Tree	Easy
110	Balanced Binary Tree	Easy
111	Minimum Depth of Binary Tree	Easy
112	Path Sum	Easy
113	Path Sum II	Medium
114	Flatten Binary Tree to Linked List	Medium
116	Populating Next Right Pointers in Each Node	Medium
117	Populating Next Right Pointers in Each Node II	Medium
124	Binary Tree Maximum Path Sum	Hard
129	Sum Root to Leaf Numbers	Medium
144	Binary Tree Preorder Traversal	Medium
145	Binary Tree Postorder Traversal	Hard
173	Binary Search Tree Iterator	Medium
199	Binary Tree Right Side View	Medium
226	Invert Binary Tree	Easy
230	Kth Smallest Element in a BST	Medium
341	Flatten Nested List Iterator	Medium