83.删除排序链表中重复的结点

题目描述：给定一个排序链表，删除所有重复的元素，使得每个元素只出现一次。
示例 1:
输入: 1->1->2
输出: 1->2

• 双指针遍历：使用两个指针分别指向当前结点和当前结点的前一个结点，方便删除操作

//C

struct ListNode* deleteDuplicates(struct ListNode* head){
    //注意这里对head是否为空的判断
    if(head == NULL || head -> next == NULL) return head;
    struct ListNode *pre, *cur;
    pre = head;
    cur = head -> next;
    while(cur != NULL){
        if(pre -> val == cur -> val){
            struct ListNode *p;
            pre -> next = cur -> next;
            p = cur;
            free(p);
            cur = pre -> next;
        }
        else{
            pre = pre -> next;
            cur = cur -> next;
        }
    }
    return head;
}

• 单指针遍历：

//JS

var deleteDuplicates = function(head) {
    let cur = head;
    while(cur && cur.next){
        if(cur.val == cur.next.val){
            cur.next = cur.next.next;
        }
        else{
            cur = cur.next;
        }
    }
    return head;
};

• 哈希表或者Map记录出现过的结点元素：这里新建了一个头结点，方便遍历

//JS

var deleteDuplicates = function(head) {
    let hash = new Map(), p = head;
    let Lhead = new ListNode();
    let q = Lhead;
    Lhead.next = head;

    while(p){
        if(hash.has(p.val)) {
            q.next = p.next;
        }     
        else {
            hash.set(p.val, 1);  
            q = q.next;
        }
        p = p.next;
    }
    p = Lhead.next;
    Lhead = null;
    return p;
};