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}
 */

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

var removeNthFromEnd = function(head, n) {
    if (head === null) {
        return null;
    }

    var p1 = p2 = head;

    //p1先移动n次
    for (var i = 0; i < n; i++) {
        p1 = p1.next;
    }

    //要移除的是第一个元素
    if (p1 === null) {
        return head.next;
    }

    //当p1指向最后一个结点，则p2.next即为要删除的结点
    while (p1.next !== null) {
        p1 = p1.next;
        p2 = p2.next;
    }

    //删除倒数第n个结点
    p2.next = p2.next.next;

    return head;
};