leetcode 114. 二叉树展开为链表

2023.10.22

        本题第一反应就是将 原二叉树的节点值 用先序遍历的方式保存到一个集合数组中。然后再重新构造出新的二叉树。 java代码如下：

/**
 * 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 {
    private List list = new ArrayList<>();
    private void preTravelsal(TreeNode cur){
        if(cur == null) return;
        list.add(cur.val);
        preTravelsal(cur.left);
        preTravelsal(cur.right);
    }
    public void flatten(TreeNode root) {
        if(root == null) return;
        preTravelsal(root);
        for(int i=1; i