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。 

input：l1 = [1,2,4], l2 = [1,3,4]
output：[1,1,2,3,4,4]

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode mergeList = new ListNode(0);
        ListNode temp = mergeList;
        while(list1 != null && list2 != null){
            if(list1.val<=list2.val){
                temp.next=list1;
                list1=list1.next;
            }else{
                temp.next=list2;
                list2=list2.next;
            }
            temp=temp.next;
        }
        if(list1 == null) temp.next=list2;
        if(list2 == null) temp.next=list1;

        return mergeList.next;
    }
}