【2021秋冬】【剑指offer】36. 二叉搜索树与双向链表

能想到是中序遍历，但是链表的构造没有想到
题解是类似指针标记

class Solution {
    Node pre,head;
    public Node treeToDoublyList(Node root) {
        if(root == null) return null;
       dfs(root);
       head.left = pre;
       pre.right = head;
       return head;
    }
    void dfs(Node root){
        if(root == null) return ;
        dfs(root.left);
        if(pre == null) head = root;
        else pre.right = root;
        root.left = pre;
        pre = root;
        
        dfs(root.right);

    }
}