Given a tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only 1 right child.

Example 1:
Input: [5,3,6,2,4,null,8,1,null,null,null,7,9]

       5
      / \
    3    6
   / \    \
  2   4    8
 /        / \ 
1        7   9

Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

 1
  \
   2
    \
     3
      \
       4
        \
         5
          \
           6
            \
             7
              \
               8
                \
                 9

还是那句话，遇到tree相关的题，先考虑递归。这道题可以分别对左右子树递归调用，然后按照要求的顺序拼接。值得注意的一件事情是，如果左子树为null，不能直接返回root，一定要先root.right = right再返回，因为right可能已经不是之前的TreeNode了。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode increasingBST(TreeNode root) {
        if(root == null)
            return null;

        TreeNode left = increasingBST(root.left);
        TreeNode right = increasingBST(root.right);

        if(left == null) {
            root.right = right;
            return root;
        }

        TreeNode p = left;
        while(p.right != null) 
            p = p.right;
        p.right = root;
        root.left = null;
        root.right = right;

        return left;
    }
}