Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

// ListNode {val: 3,next: ListNode { val: 2, next: ListNode { val: 4, next: null } } }
// ListNode {val: 1,next: ListNode { val: 3, next: ListNode { val: 4, next: null } } }
var mergeTwoLists = function(l1, l2) {
    var mergeHead = {val:-1,next:null},
        crt = mergeHead;
    while(l1 && l2){
        if(l1.val > l2.val){
            crt.next = l2;
            l2 = l2.next
        }else{
            crt.next = l1;
            l1 = l1.next
        }
        crt = crt.next;
    }
    crt.next = l1 || l2;
    return mergeHead.next
    
};