LeetCode(114)——Flatten Binary Tree to Linked List

題目：

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

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

 

二叉樹轉換爲右節點串聯起來的鏈表。對於每個節點，左節點是已經轉換完成的鏈表，只需要把左邊的鏈表插入到右邊節點之前就可以了。

AC:

public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        
        flatten(root.left);
        flatten(root.right);
        
        if (root.left != null) {
            TreeNode rightHead = root.right;
            
            root.right = root.left;
            
            TreeNode tmp = root.right;
            while (tmp.right != null) {
                tmp = tmp.right;
            }
            
            tmp.right = rightHead;
            
            root.left = null;
        }
    }