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。 

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示例 1：

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

示例 2：

输入：l1 = [], l2 = []
输出：[]

示例 3：

输入：l1 = [], l2 = [0]
输出：[0]

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class Solution {
public:
    ListNode* mergeTwoLists(ListNode* head1, ListNode* head2) {
        if(head1 == NULL || head2 == NULL) return head1 == NULL ? head2 : head1;
        ListNode* head = head1->val <= head2->val ? head1:head2;
        ListNode* cur1 = head->next;
        ListNode* cur2 = head == head1 ? head2 : head1;
        ListNode* pre = head;
        while(cur1 != NULL && cur2 != NULL) {
            if(cur1->val <= cur2->val) {
                pre->next = cur1;
                cur1 = cur1->next;
            } else {
                pre->next = cur2;
                cur2 = cur2->next;
            }
            pre = pre->next;
        }
        pre->next = cur1 != NULL ? cur1:cur2;
        return head;
    }
};