# 817 Linked List Components

## 817. [Linked List Components](https://leetcode.com/problems/linked-list-components/description/)

## 1. Question

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.

## 2. Implementation

**(1) HashSet**

```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> set = new HashSet();

        for (int node : G) {
            set.add(node);
        }

        int res = 0;

        while (head != null) {
            if (set.contains(head.val) && (head.next == null || !set.contains(head.next.val))) {
                ++res;
            }
            head = head.next;
        }
        return res;
    }
}
```

## 3. Time & Space Complexity

**HashSet:** 时间复杂度O(n), 空间复杂度O(n)


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