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.

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Just merge two sorted lists into one sorted list. Nothing special. 

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//codes

class Solution {

public:

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

              //check input

              if (l1 == NULL)return l2;

              if (l2 == NULL)return l1;

              //dummy node

              ListNode*dum = new ListNode(-1), *pl1 = l1, *pl2 = l2, *p = dum;

              while (pl1 != NULL && pl2 != NULL){

                     if (pl1->val<pl2->val){

                           p->next = pl1;

                           p = p->next;

                           pl1 = pl1->next;

                     }

                     else{

                           p->next = pl2;

                           p = p->next;

                           pl2 = pl2->next;

                     }

              }

              //check left nodes and connect to p

              if (pl1 != NULL)p->next = pl1;

              else p->next = pl2;

              p = dum->next;

              delete dum;

              return p;

       }

};