给定一个二叉树将它展开为一个单链表

原题

给定一个二叉树，原地将它展开为一个单链表。

思路

代码

class Solution {
    private TreeNode pre = null;

public void flatten(TreeNode root) {
    if (root == null)
        return;
    flatten(root.right);
    flatten(root.left);
    root.right = pre;
    root.left = null;
    pre = root;
}
}
//https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list/solution/xiang-xi-tong-su-de-si-lu-fen-xi-duo-jie-fa-by--26/