21. Merge Two Sorted List，合并两个已排序的单链表

Example 1:
Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:
Input: list1 = [], list2 = []
Output: []

Example 3:
Input: list1 = [], list2 = [0]
Output: [0]

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        if not list1:
            return list2
        if not list2:
            return list1
        if list1.val < list2.val:
            head = p = list1
            p1 = list1.next
            p2 = list2
        else:
            head = p = list2
            p1 = list1
            p2 = list2.next
        while p1!=None and p2!=None:
            if p1.val < p2.val:
                p.next = p = p1
                p1 = p1.next
            else:
                p.next = p = p2
                p2 = p2.next
        if p1 == None:
            p.next = p2
        else:
            p.next = p1
        return head

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) noexcept{
        if(!list1)
            return list2;
        if(!list2)
            return list1;
        ListNode *head, *p;
        if(list1->val < list2->val){
            head = p = list1;
            list1 = list1->next;
        } else {
            head = p = list2;
            list2 = list2->next;
        }
        while(list1 && list2){
            if(list1->val < list2->val){
                p = p->next = list1;
                list1 = list1->next;
            } else {
                p = p->next = list2;
                list2 = list2->next;
            }
        }
        if(!list1)
            p->next = list2;
        else
            p->next = list1;
        return head;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2){
    if(!list1)
        return list2;
    if(!list2)
        return list1;
    struct ListNode *head, *p;
    if(list1->val < list2->val){
        head = p = list1;
        list1 = list1->next;
    } else {
        head = p = list2;
        list2 = list2->next;
    }
    while(list1 && list2){
        if(list1->val < list2->val){
            p = p->next = list1;
            list1 = list1->next;
        } else {
            p = p->next = list2;
            list2 = list2->next;
        }
    }
    if(!list1)
        p->next = list2;
    else
        p->next = list1;
    return head;
}