Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,

Given 1->1->2, return 1->2.

Given 1->1->2->3->3, return 1->2->3.

Solution:

Run time complexity is O(n), constant space.

/**
 * 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) {
        if(head == null)
            return null;
        if(head.next == null)
            return head;
        
        ListNode pre = head;
        ListNode cur = head.next;
        while(cur != null) {
            if(cur.val == pre.val) {
                pre.next = cur.next;
            }else {
                pre = cur;
            }
            cur = cur.next;
        }
        return head;
    }
}