merge-two-sorted-lists （归并排序中的合并）

class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        if (l1 == NULL) return l2;
        if(l2 == NULL)return l1;
        ListNode *head = new ListNode();
        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。