題目來源：203. 移除連結串列元素

示例 1：

輸入：head = [1,2,6,3,4,5,6], val = 6
輸出：[1,2,3,4,5]

示例 2：

輸入：head = [], val = 1
輸出：[]

示例 3：

輸入：head = [7,7,7,7], val = 7
輸出：[]

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 
 
*/
/**
 * @param {ListNode} head
 * @param {number} val
 * @return {ListNode}
 */
var removeElements = function(head, val) {
    let cur = head;
    let prev = null;
    while(cur != null){
        if(cur.val === val){
            if(prev === null){
                head = head.next;
            }else{
                prev.next 
 
= cur.next;
            }
        }else{
            prev = cur;
        }
        cur = cur.next;
    }
    return head;
};

提示：

• 列表中的節點數目在範圍[0, 104]內
• 1 <= Node.val <= 50
• 0 <= val <= 50