83. Remove Duplicates from Sorted List

题目：

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.

链接： http://leetcode.com/problems/remove-duplicates-from-sorted-list/

一刷，在考虑什么时候应该使用dummy node，什么时候不应该。感觉跟第一个元素有关，如果第一个元素肯定在输出中，可以不使用dummy node。

class Solution(object):
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head:
            return head
        prev = head
        current = head.next
        
        while current:
            if prev.val == current.val:
                prev.next = current.next
            else:
                prev = prev.next
            current = current.next
        return head

2/14/2017, Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null) return null;
        ListNode current = head;

        while (current != null) {
            if (current.next != null && current.next.val == current.val) current.next = current.next.next;
            else current = current.next;
        }
        return head;
    }
}