链表经典题
/
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode next;
* ListNode(int x) : val(x), next(NULL) {}
* };
/
class Solution {
public:
ListNode merge(ListNode l1, ListNode* l2) {
if (l1 == NULL) return l2;
if (l2 == NULL) return l1;

    ListNode* t = NULL;
    ListNode* head = NULL;
    if (l1 -> val <= l2 -> val)
    {
        t = l1;
        l1 = l1 -> next;
        head = t;
    }
    else
    {
        t = l2;
        l2 = l2 -> next;
        head = t;
    }
    while (l1 != NULL && l2 != NULL)
    {
         if (l1 -> val <= l2 -> val)
        {
            t -> next = l1;
            l1 = l1 -> next;

        }
        else
        {
            t ->next = l2;
            l2 = l2 -> next;

        }
        t = t -> next;

    }

    if (l1 == NULL)
    {
        t -> next = l2;
    }
    else 
    {
        t -> next =l1;
    }
    return head;

}

};