Formatted question description: https://leetcode.ca/all/1836.html

# 1836. Remove Duplicates From an Unsorted Linked List

## Level

Medium

## Description

Given the `head`

of a linked list, find all the values that appear **more than once** in the list and delete the nodes that have any of those values.

Return *the linked list after the deletions*.

**Example 1:**

**Input:** head = [1,2,3,2]

**Output:** [1,3]

**Explanation:** 2 appears twice in the linked list, so all 2’s should be deleted. After deleting all 2’s, we are left with [1,3].

**Example 2:**

**Input:** head = [2,1,1,2]

**Output:** []

**Explanation:** 2 and 1 both appear twice. All the elements should be deleted.

**Example 3:**

**Input:** head = [3,2,2,1,3,2,4]

**Output:** [1,4]

**Explanation:** 3 appears twice and 2 appears three times. After deleting all 3’s and 2’s, we are left with [1,4].

**Constraints:**

- The number of nodes in the list is in the range
`[1, 10^5]`

`1 <= Node.val <= 10^5`

## Solution

Loop over the linked list and use a hash map to store each value’s number of occurrences. Then create a dummy head before `head`

. Starting from the dummy head, loop over the linked list again and remove the nodes that have values with occurrences greater than 1. Finally, return the node right after the dummy head.

```
/**
* 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> map = new HashMap<Integer, Integer>();
ListNode temp = head;
while (temp != null) {
int val = temp.val;
map.put(val, map.getOrDefault(val, 0) + 1);
temp = temp.next;
}
ListNode dummyHead = new ListNode(0);
dummyHead.next = head;
temp = dummyHead;
while (temp.next != null) {
ListNode next = temp.next;
int nextVal = next.val;
if (map.get(nextVal) > 1)
temp.next = next.next;
else
temp = temp.next;
}
return dummyHead.next;
}
}
```