Flatten Binary Tree to Linked List(Medium)

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

Preorder：前序遍历

public TreeNode lastNode = null;

public void flatten(TreeNode root) {
    if (root == null) {
        return;
    }

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

    lastNode = root;
    TreeNode right = root.right;

    flatten(root.left);
    flatten(right);
}