题目：

给你二叉树的根结点 root ，请你将它展开为一个单链表：

• 展开后的单链表应该同样使用 TreeNode ，其中 right 子指针指向链表中下一个结点，而左子指针始终为 null 。
• 展开后的单链表应该与二叉树 先序遍历 顺序相同。

方法一：先前序遍历，然后再修改左右指针

class Solution {public void flatten(TreeNode root) {List<TreeNode> list = new ArrayList<TreeNode>();preorderTraversal(root, list);int size = list.size();for(int i = 1; i < size; i++) {TreeNode pre = list.get(i - 1), cur = list.get(i);pre.right = cur;pre.left = null;}}private void preorderTraversal(TreeNode root, List<TreeNode> list) {if (root == null)return;list.add(root);preorderTraversal(root.left, list);preorderTraversal(root.right, list);}
}

方法二：寻找前驱节点

class Solution {public void flatten(TreeNode root) {TreeNode curr = root;while (curr != null) {if (curr.left != null) {TreeNode next = curr.left;TreeNode predecessor = next;while (predecessor.right != null) {predecessor = predecessor.right;}predecessor.right = curr.right;curr.left = null;curr.right = next;}curr = curr.right;}}
}