class Solution {public:    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {        if (l1 == NULL) return l2;        if(l2 == NULL)return l1;         ListNode *head = new ListNode(66);        ListNode *pp = l1;        ListNode *qq = l2;        ListNode *end = head;        while (pp && qq){            if (pp->val < qq->val){                end = end->next = pp;                pp = pp->next;            }            else{                end = end->next = qq;                qq = qq->next;            }        }        if (pp){            end->next = pp;        }        else{            end->next = qq;        }        return head->next;    }};

思路：创建一个节点，谁小就放谁，肯定会有一个链先结束，则把后面的链直接连上即可。