LeetCode 817. Linked List Components

原题链接在这里：https://leetcode.com/problems/linked-list-components/

题目：

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.

题解：

这道题是找connecteced components的组数. 如果有三组连着, e.g. [0,1], [3], [5]. 返回3.

条件就是当前点的值在G中, next点不在G中, 或者为null.

Time Complexity: O(n). n 是list长度.

Space: O(m). m = G.length.

AC 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) {
        HashSet<Integer> hs = new HashSet<Integer>();
        for(int num : G){
            hs.add(num);
        }
        
        int res = 0;
        while(head != null){
            if(hs.contains(head.val) && (head.next == null || !hs.contains(head.next.val))){
                res++;
            }
            
            head = head.next;
        }
        
        return res;
    }
}