LeetCode 897 Increasing Order Search Tree

例 1:
给予树:

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

输出:

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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private TreeNode result = new TreeNode(0);
    private TreeNode dummy = result;

    public TreeNode increasingBST(TreeNode root) {
        if (root == null) {
            return null;
        }
        increasingBST(root.left);
        dummy.right = new TreeNode(root.val);
        dummy = dummy.right;
        increasingBST(root.right);
        return result.right;
    }
}