Remove Duplicates from Sorted List II

deleteDuplicates.java

/*
Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.


*/
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(head == null) return null;
        HashMap<Integer, Integer> set = new HashMap<Integer, Integer>();
        set.put(head.val, 1);
        ListNode cur = head.next;
        while(cur != null) {
            if(set.containsKey(cur.val)) {
                set.put(cur.val, set.get(cur.val) + 1);
            } else {
                set.put(cur.val, 1);
            }
            cur = cur.next;
        }
        
        cur = head;
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode prev = dummy;
        while(cur != null) {
            if(set.get(cur.val) > 1) {
                prev.next = cur.next;
                cur = cur.next;
            } else {
                prev = cur;
                cur = cur.next;
            }
        }
        
        return dummy.next;
    }
}