Flatten Binary Tree to Linked List

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

前序遍历，每层都记录当前节点的左右孩子节点，用tail记录修改后的链表结尾，并返回给上一层。在修改结构之前注意把当前节点的左右置空。

public class FlattenBinaryTreeToLinkedList {
    public void flatten(TreeNode root) {
        dfs(root);
    }

    public TreeNode dfs(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode left = root.left;
        TreeNode right = root.right;
        //将当前节点的左右孩子置空
        root.left = null;
        root.right = null;
        TreeNode tail = root;
        //连接左子树
        if (left != null) {
            tail.right = left;
            tail = dfs(left);
        }
        //连接右子树
        if (right != null) {
            tail.right = right;
            tail = dfs(right);
        }
        return tail;
    }
}