給定一個排序連結串列，刪除所有重複的元素，使得每個元素只出現一次。

示例 1:

輸入: 1->1->2
輸出: 1->2

示例 2:

輸入: 1->1->2->3->3
輸出: 1->2->3

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var deleteDuplicates = function(head) {
    if (head === null) return null;
    var nodePrev = null;
    var node = head;
    var singlePrev = head;
    var bStart = true;
    while (node !== null) {
        if (nodePrev !== null && node.val != nodePrev.val) {
            singlePrev.next = node;
            singlePrev = node;
        }
        nodePrev = node;
        node = node.next;
    }
    singlePrev.next = null;
    return head; 
};