归并方法，可与20题对照着看

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

/**
 * 
 * @param lists ListNode类一维数组 
 * @return ListNode类
 */
function mergeKLists( lists ) {
    // write code here
//     let node = {
//         val:null,
//         next:null
//     };
   if (lists.length <= 1) return lists[0] || null;
    return merge(lists, 0, lists.length-1);
}
function merge(lists, l, r) {
    if (l > r) return null;
    if (l === r) return lists[l];
    const mid = Math.floor((l+r)/2);
    return mergeTwoList(merge(lists, l, mid), merge(lists, mid+1, r));
}
function mergeTwoList (list1, list2) {
    if (!list1 || !list2) return list1 || list2;
    let p1 = list1, p2 = list2, head = {next: null}, p = head;
    while(p1 && p2) {
        if (p1.val > p2.val) {
            p.next = p2;
            p2 = p2.next;
        } else {
            p.next = p1;
            p1 = p1.next;
        }
        p = p.next;
    }
    p.next = p1 || p2;
    return head.next;
}
module.exports = {
    mergeKLists : mergeKLists
};