9-24每日一题：430. 扁平化多级双向链表

Leetcode430. 扁平化多级双向链表

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* prev;
    Node* next;
    Node* child;
};
*/

class Solution {
public:
    Node* flatten(Node* head) {
        auto res = dfs(head);
        return res[0];
    }

    vector<Node*> dfs(Node* head) {
        if (!head) return {NULL, NULL};
        auto cur = head, tail = head;
        while (cur) {
            tail = cur;
            if (cur->child) {
                auto t = dfs(cur->child);
                cur->child = NULL;
                if (cur->next) cur->next->prev = t[1];
                t[1]->next = cur->next;
                cur->next = t[0];
                t[0]->prev = cur;
            }
            else cur = cur->next;
        }
        return {head, tail};
    }
};