19. 删除链表的倒数第 N 个结点（算法200题-49）

/**
 * 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} n
 * @return {ListNode}
 */
var removeNthFromEnd = function(head, n) {
    const stack = []
    let cur = head
    while (cur) {
        stack.push(cur)
        cur = cur.next
    }
    const length = stack.length,
        pre = stack[length - n - 1] || null,
        target = stack[length - n]
    if (!length || length < n) {
        return null
    }
    if (pre === null) {
        head = head.next
    } else {
        pre.next = target.next
    }
    return head
};

/**
 * 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} n
 * @return {ListNode}
 */
var removeNthFromEnd = function (head, n) {
    let curi = head,
        curj = head
    while (n > 0) {
        curj = curj.next
        n--
    }
    if(!curj) {
        head = head.next
        return head
    }
    while (curj.next) {
        curj = curj.next
        curi = curi.next
    }
    curi.next = curi.next.next
    return head
};