[LeetCode] 21. Merge Two Sorted Lists 解题思路

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.

问题：将两个已排序的列表，合并为一个有序列表。

令 head 为两个列表表头中较小的一个，令 p 为新的已排序的最后一个元素。令 l1, l2 分别为两个列表中未排序部分的首节点。依次将 l1, l2 中的较小值追加到 p 后面，并调整 p 和 l1、l2较小者指针即可。

     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {

         if (l1 == NULL) {
             return l2;
         }

         if (l2 == NULL) {
             return l1;
         }

         ListNode* head = new ListNode();
         if (l1->val <= l2->val) {
             head = l1;
             l1 = l1->next;
         }else{
             head = l2;
             l2 = l2->next;
         }

         ListNode* p = head;

         while(l1 != NULL && l2 != NULL){

             if(l1->val <= l2->val){
                 p->next = l1;
                 p = p->next;
                 l1 = l1->next;
             }else{
                 p->next = l2;
                 p = p->next;
                 l2 = l2->next;
             }
         }

         if (l1 == NULL) {
             p->next = l2;
         }

         if (l2 == NULL) {
             p->next = l1;
         }

         return head;
     }