合并两有序链表

题目

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */


struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2)
{
      struct ListNode *prehead = (struct ListNode*)malloc(sizeof(struct ListNode));
      struct ListNode *prev = prehead;
      while(l1 != NULL && l2 != NULL)
      {
            if(l1->val < l2->val)
            {
                  prev->next = l1;
                  l1 = l1->next;
            }
            else
            {
                  prev->next = l2;
                  l2 = l2->next;
                 }
            prev = prev->next;
      }
      prev->next = (l1==NULL ? l2:l1);
      return prehead->next;
}