Leetcode Study Day 28

Copy List with Random Pointer

A linked list of length n is given such that each node contains an additional random pointer, which could point to any node in the list, or null.

Construct a deep copy of the list. The deep copy should consist of exactly n brand new nodes, where each new node has its value set to the value of its corresponding original node. Both the next and random pointer of the new nodes should point to new nodes in the copied list such that the pointers in the original list and copied list represent the same list state. None of the pointers in the new list should point to nodes in the original list.

For example, if there are two nodes X and Y in the original list, where X.random –> Y, then for the corresponding two nodes x and y in the copied list, x.random –> y.

Return the head of the copied linked list.

The linked list is represented in the input/output as a list of n nodes. Each node is represented as a pair of [val, random_index] where:

val: an integer representing Node.val
random_index: the index of the node (range from 0 to n-1) that the random pointer points to, or null if it does not point to any node.
Your code will only be given the head of the original linked list.

Example 1:


Input: head = [[7,null],[13,0],[11,4],[10,2],[1,0]]
Output: [[7,null],[13,0],[11,4],[10,2],[1,0]]
Example 2:


Input: head = [[1,1],[2,1]]
Output: [[1,1],[2,1]]
Example 3:



Input: head = [[3,null],[3,0],[3,null]]
Output: [[3,null],[3,0],[3,null]]
 

Constraints:

0 <= n <= 1000
-104 <= Node.val <= 104
Node.random is null or is pointing to some node in the linked list.

My solution

My solution is to use an unordered_map to store the original node and the copied node.

Firstly, we store the original node and a new node with the same value as the original one as the copied node.

unordered_map<Node*, Node*> nodeMap;
    Node * current = head;
    while(current){
        // copy the val from the original to a new one
        nodeMap[current] = new Node(current->val);
        current = current -> next;
    }

Secondly, we iterate the linked list from head again, and this time, we set the next pointer and the random pointer of the copied node by looking up the original node in the unordered_map.

current = head;
while(current){
    if(current->next){
        nodeMap[current]->next = nodeMap[current->next];
    }
    if(current->random){
        nodeMap[current]->random = nodeMap[current->random];
    }
    current = current -> next;
}
return nodeMap[head];

Full code

/*
// Definition for a Node.
class Node {
public:
    int val;
    Node* next;
    Node* random;
    
    Node(int _val) {
        val = _val;
        next = NULL;
        random = NULL;
    }
};
*/

class Solution {
public:
    Node* copyRandomList(Node* head) {
        unordered_map<Node*, Node*> nodeMap;
        Node * current = head;
        while(current){
            // copy the val from the original to a new one
            nodeMap[current] = new Node(current->val);
            current = current -> next;
        }
        current = head;
        while(current){
            if(current->next){
                nodeMap[current]->next = nodeMap[current->next];
            }
            if(current->random){
                nodeMap[current]->random = nodeMap[current->random];
            }
            current = current -> next;
        }
        return nodeMap[head];
    }
};