展平多层双向链表

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:

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Input:
 1---2---3---4---5---6--NULL
         |
         7---8---9---10--NULL
             |
             11--12--NULL

Output:
1-2-3-7-8-11-12-9-10-4-5-6-NULL

Given the following multilevel doubly linked list:

We should return the following flattened doubly linked list:

Solution

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const flatten = function(head) {
  let currentNode = head;
  let prevNode = null;

  while (currentNode) {
    if (currentNode.child) {
      let nextNode = currentNode.next;
      currentNode.child.prev = currentNode;
      currentNode.next = flatten(currentNode.child);
      currentNode.child = null;

      if (nextNode) {
        prevNode.next = nextNode;
        nextNode.prev = prevNode;
      }
    }

    prevNode = currentNode;
    currentNode = currentNode.next;
  }

  return head;
}