add leetcode.tree
add valid sudoku
add tree node
add same tree
add symmetric tree
add validate binary search tree
add path sum
add path sum ii
add RecoverBinarySearchTree
add BinaryTreeInorderTraversal
add UniqueBinarySearchTrees
add solution
add MaximumDepthOfBinaryTree
add MinimumDepthOfBinaryTree
add BalancedBinaryTree
add FlattenBinaryTreeToLinkedList
add BinaryTreeLevelOrderTraversalII
ass BinaryTreeLevelOrderTraversal
add BinaryTreeZigzagLevelOrderTraversal
transfer unique bst II from recursion to tree package