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Copy List with Random Pointer

ID: 105; medium; 复制带随机指针的链表

Solution 1 (Java)

/**
* Definition for singly-linked list with a random pointer.
* class RandomListNode {
* int label;
* RandomListNode next, random;
* RandomListNode(int x) { this.label = x; }
* };
*/
public class Solution {
/**
* @param head: The head of linked list with a random pointer.
* @return: A new head of a deep copy of the list.
*/
public RandomListNode copyRandomList(RandomListNode head) {
if (head == null) return null;
RandomListNode dummy = new RandomListNode(0);
RandomListNode prev = dummy, newNode;
HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
while (head != null) {
// deal with the core nodes
if (!map.containsKey(head)) {
newNode = new RandomListNode(head.label);
map.put(head, newNode);
} else {
newNode = map.get(head);
}
prev.next = newNode;
// deal with random pointers of the core nodes
if (head.random != null) {
if (!map.containsKey(head.random)) {
newNode.random = new RandomListNode(head.random.label);
map.put(head.random, newNode.random);
} else {
newNode.random = map.get(head.random);
}
}
prev = newNode;
head = head.next;
}
return dummy.next;
}
}

Notes

  • This solution uses a HashMap to keep track of the original nodes and their duplicates.
  • For each iteration, we first deal with the node itself. If it is not in the HashMap, we add it to the map where the key is itself and the value is its duplicate. If it is in the map, the next node for the deep copy is its value, i.e., the duplicate in the map. Next, we deal with the random pointers of the node. Using a similar approach, we point the random pointer of the node in the deep copy to a duplicate.
  • Do not forget to update prev and head. Here, prev is for constructing the new deep copy and head is just for traversing the original linked list.
  • Time complexity is O(n) and space complexity is also O(n).

Scratches

Solution 2 (Java)

/**
* Definition for singly-linked list with a random pointer.
* class RandomListNode {
* int label;
* RandomListNode next, random;
* RandomListNode(int x) { this.label = x; }
* };
*/
public class Solution {
/**
* @param head: The head of linked list with a random pointer.
* @return: A new head of a deep copy of the list.
*/
public RandomListNode copyRandomList(RandomListNode head) {
if (head == null) return null;
copyNext(head);
copyRandom(head);
return splitLists(head);
}
private void copyNext(RandomListNode head) {
RandomListNode newNode = null;
while (head != null) {
newNode = new RandomListNode(head.label);
newNode.next = head.next;
// newNode.random = head.random;
head.next = newNode;
head = head.next.next;
}
}
private void copyRandom(RandomListNode head) {
while (head != null) {
if (head.random != null) {
head.next.random = head.random.next;
}
head = head.next.next;
}
}
private RandomListNode splitLists(RandomListNode head) {
RandomListNode newHead = head.next;
while (head != null) {
RandomListNode duplicate = head.next;
head.next = head.next.next;
if (duplicate.next != null) {
duplicate.next = duplicate.next.next;
}
head = head.next;
}
return newHead;
}
}

Notes

  • This solution uses three pass to copy the original list. Thus, there are three steps:
    1. 1.
      copyNext: This step creates new duplicated nodes and point their next pointers to their originals'.
    2. 2.
      copyRandom: This step simply point the duplicates' random pointers to the corresponding duplicates of their original nodes' random pointers. The key operation here is head.next.random = head.random.next. The left is the random pointer of the duplicate and the right is the duplicate of the original random node.
    3. 3.
      splitLists: We cut the connections between the original list and the new deep copy. Be careful with null checking in the loop to prevent NullPointerException.
  • The time complexity is still O(n), but the space complexity now is O(1).

Scratches