83. Remove Duplicates from Sorted List

问题
Given the head of a sorted linked list, delete all duplicates such that each element appears only once. Return the linked list sorted as well.

设置前一个节点和当前节点两个指针。
由于是有数的链表，遍历时可以直接比较两个节点。
如相等则前一个节点的next指向当前节点的next。

/**
 * 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 deleteDuplicates(ListNode head) {
        
        if ( head == null){
            return head;
        }
        if ( head.next == null){
            return head;
        }
        ListNode prev = head;
        ListNode curr = head.next;
        
        while(curr != null){
            if(prev.val != curr.val){
                curr = curr.next;
                prev = prev.next;
            }
            else{
                prev.next = curr.next;
                curr = curr.next;
            }
        }
        return head;
    }
}