【LeetCode算法練習（C++）】Merge Two Sorted Lists

題目：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.

連接：Merge Two Sorted Lists
解法：比較兩鏈表頭值大小，將小者插入新鏈表尾，若任一鏈表爲空，直接將另一鏈表插入新鏈表尾。時間O(m+n)

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *head = new ListNode(0);
        head->next = NULL;
        ListNode *p = head;
        while (l1 || l2) {
            ListNode *tmp = new ListNode(0);
            tmp->next = NULL;
            if (!l1 && l2) {
                tmp->val = l2->val;
                l2 = l2->next;
            } else if (!l2 && l1) {
                tmp->val = l1->val;
                l1 = l1->next;
            } else if (l1->val > l2->val) {
                tmp->val = l2->val;
                l2 = l2->next;
            } else {
                tmp->val = l1->val;
                l1 = l1->next;
            }
            p->next = tmp;
            p = p->next;
        }
        p = head;
        head = head->next;
        p->next = NULL;
        delete(p);
        return head;
    }
};

Runtime: 9 ms