Remove Duplicates from Sorted List.java

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param ListNode head is the head of the linked list
     * @return: ListNode head of linked list
     */
    public static ListNode deleteDuplicates(ListNode head) { 
        // write your code here
        if (head == null || head.next == null) {
            return head;
        }
        
        
        ListNode pre = head;
        ListNode curr = head.next;
        
        while (curr != null) {
            if (pre.val == curr.val) {
                curr = curr.next;
            } else {
                pre.next = curr;
                pre = pre.next;
            }
        }
        pre.next = null;
        
        return head;
    }  
}