Binary Tree Divide & Conquer
### Closest Binary Search Tree Value
Vertical Order Traversal of a Binary Tree
Minimum Subtree
Binary Tree Paths
Flatten Binary Tree to Linked List
Balanced Binary Tree
Kth Smallest Element in a BST
Lowest Common Ancestor III
Validate Binary Search Tree
Closest Binary Search Tree Value II
Binary Search Tree Iterator
Find tree value(在binary tree中search a value)
**Bottom-up 必背代码
public boolean findTarget(TreeNode root, int target) {
if (root == null) return false;
if (root.val == target) return true;
boolean left = findTarget(root.left, target);
boolean right = findTarget(root.right, target);
return left || right;
}
Maximum Average Subtree
在java里面如果想同时返回多个值，唯一的办法是用一个自己创建的object(e.g. Pair)
