Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

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

        if (root == null){
            return;
        }
        //先遍历到6
        Flatten(root.right);
        Flatten(root.left);

        //设置6的right指向LastTreeNode which is null
        root.right = LastTreeNode;
        //设置6的left指向 null
        root.left = null;
        //把LastTreeNode 设置成6
        LastTreeNode = root;

    }
}