c_cpp 合并两个排序列表，递归和迭代

Posted 2021-05-14

ListNode* merge_two_sorted_lists_recursive(ListNode *L1, ListNode *L2) {
    if(!L1) return L2;
    if(!L2) return L1;
    ListNode *head = NULL;
    if(L1->val < L2->val) {
        head = L1;
        head->next = merge_two_sorted_lists_recursive(L1->next, L2);
    }
    else {
        head = L2;
        head->next = merge_two_sorted_lists_recursive(L1, L2->next);
    }
    return head;
}

ListNode* merge_two_sorted_lists_iterative(ListNode *L1, ListNode *L2) {
    if(!L1) return L2;
    if(!L2) return L1;
    ListNode *p1 = L1, *p2 = L2;
    ListNode *head = NULL;
    if(p1->val < p2->val) {
        head = p1;
        p1 = p1->next;
    } else {
        head = p2;
        p2 = p2->next;
    }
    
    ListNode *tail = head;
    while(p1 && p2) {
        if(p1->val < p2->val) {
            tail->next = p1;
            p1 = p1->next;
        } else {
            tail->next = p2;
            p2 = p2->next;
        }
        tail = tail->next;
    }
    /* wrong
    if(p1) tail->next = p1;
    if(p2) tail->next = p2;
    tail->next = NULL; */
    
    if(p1) tail->next = p1;
    else tail->next = p2;
    
    return head;
}