LeetCode:Merge Two Sorted Lists

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.

思路：增加一个头指针，依次加入l1，l2首元素中值小的元素

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
    public:
        ListNode *mergeTwoLists(ListNode *l1,ListNode *l2) {
            ListNode *head;
            head->next=NULL;
            ListNode *p=head;
            while(l1!=NULL&&l2!=NULL)
            {
                if(l1->val<l2->val)
                {
                    p->next=l1;
                    l1=l1->next;
                }
                else
                {
                    p->next=l2;
                    l2=l2->next;
                }
                p=p->next;
            }
            if(l1!=NULL)
                p->next=l1;
            if(l2!=NULL)
                p->next=l2;
            return head->next;
        }
};