21.Merge Two Sorted Lists

[niuworld]

Question:

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.

Solution:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *preheader = new ListNode(0), *po = preheader;
         if ( l1 == NULL ) return l2;
         if ( l2 == NULL ) return l1;
         while( l1 && l2 ){
         if ( l2 -> val < l1 -> val )
         { //p -> next = new ListNode(l2 -> val);
          po -> next = l2;
            l2 = l2 -> next;

         }
           else {
             po -> next = l1;
               l1 = l1 -> next;

           }
           po = po -> next;
         }
         if ( l1 ) po -> next = l1;
         if ( l2 ) po -> next = l2;

         return preheader -> next;
    }
};