题解 | #二叉树展开为单链表 在前序遍历的基础上做#

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 {
    /**
     * 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
     *
     * 
     * @param root TreeNode类 
     */
    public void expandTree (TreeNode root) {
        if (root == null){
            return;
        }
        ArrayList<TreeNode> nodeList = new ArrayList<>();
        process(root, nodeList);
        for (int i = 0; i < nodeList.size() - 1; i++) {
            TreeNode pre = nodeList.get(i);
            TreeNode cur = nodeList.get(i + 1);
            pre.left = null;
            pre.right = cur;
        }
    }
    public static void process(TreeNode root, ArrayList<TreeNode> nodelist){
        if (root == null){
            return;
        }
        nodelist.add(root);
        process(root.left, nodelist);
        process(root.right, nodelist);
    }
}