Depth-First Search

1.Easy

(101)Symmetric Tree

(104)Maximum Depth of Binary Tree

(226)Invert Binary Tree

(543)Diameter of Binary Tree

(617)Merge Two Binary Trees

2.Medium

(94)Binary Tree Inorder Traversal

(98)Validate Binary Search Tree

(114)Flatten Binary Tree to Linked List

(200)Number of Islands

(207)Course Schedule

(337)House Robber III

(399)Evaluate Division

(437)Path Sum III

(538)Convert BST to Greater Tree

3.Hard

(124)Binary Tree Maximum Path Sum

(297)Serialize and Deserialize Binary Tree