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LeetCode 114. 二叉树展开为链表，难度中等。

DFS

解题思路：先用 DFS 遍历二叉树，将树的结果存放在 List 中，然后更新原先的二叉树。

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public void preorderTraversal(List<TreeNode> traList, TreeNode root) {if (root == null) {return;}traList.add(root);preorderTraversal(traList, root.left);preorderTraversal(traList, root.right);}public void flatten(TreeNode root) {List<TreeNode> traList = new ArrayList<>();preorderTraversal(traList, root);TreeNode temp = root;for (int i = 1; i < traList.size(); ++i) {temp.right = traList.get(i);temp.left = null;temp = temp.right;}}
}