[LeetCode] #19 Remove Nth Node From End of List

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
var removeNthFromEnd = function(head, n) {
    var dummy = new ListNode(-1);
    dummy.next = head;
    
    var fast = head;
    var slow = head;
    var pre = dummy;
    
    for (; n > 0; n--) {
        fast = fast.next;
    }
    while (fast != null) {
        fast = fast.next;
        pre = slow;
        slow = slow.next;
    }
    pre.next = slow.next;
    
    return dummy.next;
    
};