题解 | #删除链表的倒数第n个节点#

/*
 * function ListNode(x){
 *   this.val = x;
 *   this.next = null;
 * }
 */

/**
  * 
  * @param head ListNode类 
  * @param n int整型 
  * @return ListNode类
  */
function removeNthFromEnd( head ,  n ) {
    if (head === null) return head;
  
    let fast = head;
    // 删除了链表的倒数第 n 个节点
    while (fast && n > 0) {
        fast = fast.next;
        n--;
    }
  
    if (n > 0) return null;
    
    let slow = head;
    let pre = null; // before nth node
    
    while (fast) {
        pre = slow;
        fast = fast.next;
        slow = slow.next;
    }
    
    if (pre === null) return slow.next;
    
    pre.next = slow.next; 
    
    return head;    
}
module.exports = {
    removeNthFromEnd : removeNthFromEnd
};