21.合并两个有序链表

/**
 * 将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */

function ListNode(val, next) {
    this.val = (val === undefined ? 0 : val)
    this.next = (next === undefined ? null : next)
}

/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function (list1, list2) {
    let prehead = new ListNode(-1)
    let prev = prehead
    while(list1 !== null || list2!==null) {
        if(list1 === null) {
            prev.next = list2
            break
        }
        if(list2 === null) {
            prev.next = list1
            break
        }
        if(list1.val <= list2.val) {
            prev.next = list1
            list1 = list1.next
        } else {
            prev.next = list2
            list2 = list2.next
        }
        prev = prev.next
    }
    return prehead.next
};

var mergeTwoLists = function (list1, list2) {
    if(list1 === null) {
        return list2
    } else if(list2 === null) {
        return list1
    } else if (list1.val <=list2.val) {
        list1.next = mergeTwoLists(list1.next, list2)
        return list1
    } else {
        list2.next = mergeTwoLists(list1, list2.next)
        return list2
    }
};

// 输入：l1 = [1,2,4], l2 = [1,3,4]
// 输出：[1,1,2,3,4,4]