Merge Two Sorted Lists <LeetCode>

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Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists. 合并两个排好序的链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        if(l1==NULL||l2==NULL)
        {
            if(NULL!=l1)  return l1;
            else return l2;
        }
        else 
        {
            ListNode *head;
            if(l1->val<=l2->val)   head=l1;
            else head=l2;
            ListNode *up=head;
            ListNode *list1=l1;
            ListNode *list2=l2;
            while(list1!=NULL&&list2!=NULL)
            {
                ListNode *temp;
                if(list1->val<=list2->val)
                {
                    temp=list1;
                    list1=list1->next;
                }
                else
                {
                    temp=list2;
                    list2=list2->next;
                }
                up->next=temp;
                up=temp;
            }
            if(list1!=NULL||list2!=NULL)
            {
                if(list1!=NULL)
                {
                    up->next=list1;
                }
                else  up->next=list2;
            }
            return head;
        }
    }
};