You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.
Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.
Example 1:
Input: head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12] Output: [1,2,3,7,8,11,12,9,10,4,5,6] Explanation: The multilevel linked list in the input is as follows: After flattening the multilevel linked list it becomes:
Example 2:
Input: head = [1,2,null,3] Output: [1,3,2] Explanation: The input multilevel linked list is as follows: 1---2---NULL | 3---NULL
Example 3:
Input: head = [] Output: []
How multilevel linked list is represented in test case:
We use the multilevel linked list from Example 1 above:
1---2---3---4---5---6--NULL | 7---8---9---10--NULL | 11--12--NULL
The serialization of each level is as follows:
[1,2,3,4,5,6,null] [7,8,9,10,null] [11,12,null]
To serialize all levels together we will add nulls in each level to signify no node connects to the upper node of the previous level. The serialization becomes:
[1,2,3,4,5,6,null] [null,null,7,8,9,10,null] [null,11,12,null]
Merging the serialization of each level and removing trailing nulls we obtain:
[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
Constraints:
- Number of Nodes will not exceed 1000.
1 <= Node.val <= 10^5
A:
就是一个个递归,然后插入即可。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 | /* // Definition for a Node. class Node { public: int val; Node* prev; Node* next; Node* child; }; */ class Solution { public: Node* flatten(Node* head) { Node* runner = head; while(runner!=nullptr){ if(runner->child){ Node* subHead = flatten(runner->child); runner->child = nullptr; // insert subHead after runner Node* nextNode = runner->next; runner->next = subHead; subHead->prev = runner; while(runner->next){ // till the end runner=runner->next; } runner->next = nextNode; if(nextNode) nextNode->prev = runner; } runner = runner->next; } return head; } }; |
Mistakes:
28行, 没有检查 nextNode
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