19. 删除链表的倒数第 N 个结点 & 43. 字符串相乘

输入：head = [1,2,3,4,5], n = 2
输出：[1,2,3,5]

/**
 * 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) {
    // 哨兵节点
    let dump = new ListNode();
    dump.next = head;
    // 快慢指针
    let fast = slow = dump;

    // 快指针先走n步
    while(n--) {
        fast = fast.next;
    }
    // 快指针走到最后，当前slow为倒数第n+1个节点
    while(fast && fast.next) {
        fast = fast.next;
        slow = slow.next;
    }
    slow.next = slow.next.next;
    return dump.next;
};

输入: num1 = "2", num2 = "3"
输出: "6"

输入: num1 = "123", num2 = "456"
输出: "56088"

/**
 * @param {string} num1
 * @param {string} num2
 * @return {string}
 */
let multiply = function(num1, num2) {
    if(num1 === '0' || num2 === '0') return "0"
    let res = []
    for(let i = 0; i < num1.length; i++){
        let tmp1 = +num1[num1.length - 1 - i]
        for(let j = 0; j < num2.length; j++){
            let tmp2 = +num2[num2.length - 1 - j]
            let pos = res[i + j] ? res[i + j] + tmp1 * tmp2 : tmp1 * tmp2
            res[i + j] = pos % 10
            pos >=10 && (res[i + j + 1] = res[i + j + 1] ? res[i + j + 1] + Math.floor(pos / 10) : Math.floor(pos / 10));
        }
    }
    return res.reverse().join("");
}