leetcode二叉树114

/**
<p>给你二叉树的根结点 <code>root</code> ，请你将它展开为一个单链表：</p>

<ul>
	<li>展开后的单链表应该同样使用 <code>TreeNode</code> ，其中 <code>right</code> 子指针指向链表中下一个结点，而左子指针始终为 <code>null</code> 。</li>
	<li>展开后的单链表应该与二叉树 <a href="https://baike.baidu.com/item/%E5%85%88%E5%BA%8F%E9%81%8D%E5%8E%86/6442839?fr=aladdin" target="_blank"><strong>先序遍历</strong></a> 顺序相同。</li>
</ul>

<p> </p>

<p><strong>示例 1：</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/01/14/flaten.jpg" style=" 500px; height: 226px;" />
<pre>
<strong>输入：</strong>root = [1,2,5,3,4,null,6]
<strong>输出：</strong>[1,null,2,null,3,null,4,null,5,null,6]
</pre>

<p><strong>示例 2：</strong></p>

<pre>
<strong>输入：</strong>root = []
<strong>输出：</strong>[]
</pre>

<p><strong>示例 3：</strong></p>

<pre>
<strong>输入：</strong>root = [0]
<strong>输出：</strong>[0]
</pre>

<p> </p>

<p><strong>提示：</strong></p>

<ul>
	<li>树中结点数在范围 <code>[0, 2000]</code> 内</li>
	<li><code>-100 <= Node.val <= 100</code></li>
</ul>

<p> </p>

<p><strong>进阶：</strong>你可以使用原地算法（<code>O(1)</code> 额外空间）展开这棵树吗？</p>
<div><div>Related Topics</div><div><li>栈</li><li>树</li><li>深度优先搜索</li><li>链表</li><li>二叉树</li></div></div><br><div><li> 1168</li><li> 0</li></div>
*/

//leetcode submit region begin(Prohibit modification and deletion)

import java.util.ArrayList;

/**
 * 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 flatten(TreeNode root) {
		List<TreeNode> list = new ArrayList<>();
		preOrder(root,list);
		for (int i = 1; i < list.size(); i++) {
			TreeNode prev= list.get(i-1);
			TreeNode curr = list.get(i);
			prev.left = null;
			prev.right = curr;
		}
    }

	void preOrder(TreeNode root,List<TreeNode> list){
		if(root == null ){
			return ;
		}
		list.add(root);
		preOrder(root.left,list);
		preOrder(root.right,list);
	}
}
//leetcode submit region end(Prohibit modification and deletion)