题解 | #牛群的树形结构展开#

思路：

最简单的办法自然是得到二叉树的前序遍历结果，然后按顺序新建链表即可。

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    List<Integer> list = new ArrayList<>();
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     * 0 --- 3000
     * preOrder
     * left == null
     * @param root TreeNode类 
     * @return TreeNode类
     */
    public TreeNode flattenTree (TreeNode root) {
        if(root == null) return null;
        preOrder(root);
        TreeNode head = new TreeNode(list.get(0));
        TreeNode node = head;
        for(int i=1;i<list.size();i++){
            node.right = new TreeNode(list.get(i));
            node = node.right;
        }
        return head;
    }

    void preOrder(TreeNode root){
        if(root == null) return;
        list.add(root.val);
        if(root.left != null) preOrder(root.left);
        if(root.right != null) preOrder(root.right);
    }
}