Remove Duplicates From an Unsorted Linked List

给一个链表, 删除里面所有重复的元素, 链表本身不是已排序的.

因为没有排序, 所以要用set查重. 然后新建个链表返回即可.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {

    public ListNode deleteDuplicatesUnsorted(ListNode head) {
        Map<Integer, Integer> count = new HashMap<>();
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        while(dummy.next != null) {
            count.put(dummy.next.val, count.getOrDefault(dummy.next.val, 0) + 1);
            dummy = dummy.next;
        }
        ListNode dn = new ListNode(0); 
        ListNode res = dn;
        while(head != null) {
            if(count.get(head.val) <= 1){
                dn.next = new ListNode(head.val);
                dn = dn.next;
            }
            head = head.next;
        }
        return res.next;
    }
}