82. Remove Duplicates from Sorted List II

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.

删掉重复的节点，这里考虑好特殊情况就好
使用3个指针，指向已经确定没有重复的末尾，正在检查的元素和找重复范围的元素。

var deleteDuplicates = function(head) {
    if (!head)
        return head;
    var dummy = new ListNode(0);
    dummy.next = head;
    var pre = dummy;
    var now = head;
    var exam = head.next;
    while (exam) {
        if (now.val === exam.val) {
            while (exam && now.val === exam.val) 
                exam = exam.next;
            pre.next = exam;
            if (exam === null) break;
            now = exam;
            exam = exam.next;
        } else {
            now = now.next;
            exam = exam.next;
            pre = pre.next;
        }
    }
    return dummy.next;
};