来源：力扣（LeetCode） 链接：https://leetcode.cn/problems/merge-two-sorted-lists 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

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。

解答

/**
 * Definition for singly-linked list.
 * 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 first = null,
        pre = first,
        point1 = list1,
        point2 = list2;
    while (point1) {
        if (point2) {
            if (point1.val <= point2.val) {
                if (first === null) {
                    first = point1;
                }
                if (pre) {
                    pre.next = point1;
                }
                pre = point1;
                point1 = point1.next;
            } else {
                if (first === null) {
                    first = point2;
                }
                if (pre) {
                    pre.next = point2;
                }
                pre = point2;
                point2 = point2.next;
            }
        } else {
            if (first === null) {
                first = point1;
            }
            if (pre) {
                pre.next = point1;
            }
            pre = point1;
            point1 = point1.next;
        }
    }
    while (point2) {
        if (first === null) {
            first = point2;
        }
        if (pre) {
            pre.next = point2;
        }
        pre = point2;
        point2 = point2.next;
    }
    return first;
};