Leetcode 817. Linked List Components

We are given head, the head node of a linked list containing unique integer values.

We are also given the list G, a subset of the values in the linked list.

Return the number of connected components in G, where two values are connected if they appear consecutively in the linked list.

Example 1:

Input: 
head: 0->1->2->3
G = [0, 1, 3]
Output: 2
Explanation: 
0 and 1 are connected, so [0, 1] and [3] are the two connected components.

Example 2:

Input: 
head: 0->1->2->3->4
G = [0, 3, 1, 4]
Output: 2
Explanation: 
0 and 1 are connected, 3 and 4 are connected, so [0, 1] and [3, 4] are the two connected components.

Note:

• If N is the length of the linked list given by head, 1 <= N <= 10000.
• The value of each node in the linked list will be in the range [0, N - 1].
• 1 <= G.length <= 10000.
• G is a subset of all values in the linked list.

Analysis:

Scanning through the list, if node.val is in G and node.next.val isn't (including if node.next is null), then this must be the end of a connected component.

Code (Java):

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public int numComponents(ListNode head, int[] G) {
        Set<Integer> setG = new HashSet<>();
        
        // insert each number in G into the hash set
        //
        for (int num : G) {
            setG.add(num);
        }
        
        int numCC = 0;
        
        // Iterate all nodes in the linked list and get number of cc
        //
        while (head != null) {
            if (setG.contains(head.val) && 
                (head.next == null || !setG.contains(head.next.val))) {
                numCC++;
            }
            
            head = head.next;
        }
        
        return numCC;
     }
}