【C++】鏈表的歸併排序merge

/**
 * 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) {}
 * };
 */
ListNode* merge(ListNode *head1, ListNode *head2) {
        if (head1 == nullptr){
            return head2;
        }
        if (head2 == nullptr){
            return head1;
        }
        
        ListNode *head;
        ListNode *p1 = head1;
        ListNode *p2 = head2;
        if (head1->val < head2->val){
            head = head1;
            p1 = p1->next;
        }else{
            head = head2;
            p2 = p2->next;
        }
        
        ListNode *cur = head;
        while (p1 && p2){
            if (p1->val < p2->val){
                cur->next = p1;
                p1 = p1->next;
            }else{
                cur->next = p2;
                p2 = p2->next;
            }
            cur = cur->next;
        }
        
        if (p1){
            cur->next = p1;
        }else{
            cur->next = p2;
        }
        
        return head;
    }