validate binary search tree
RecoverBinaryTree
Same Tree
Symmetric Tree
Binary Tree Level Traversal
ZigZag Traversal
max depth
Balance Tree
Minium Depth
SortedArrayTo Binary Search Tree
SorteList2BST
Flatten Binary Tree
Maximum Binary Tree Sum
PathSum
PathSumII
Distinct SubSequence
Next Right Pointer
Populate next level
Pascal Triangle
pascal triangle ii