21. Merge Two Sorted Lists

Well, this is a real easy one.

1:  /**  
2:   * Definition for singly-linked list.  
3:   * struct ListNode {  
4:   *   int val;  
5:   *   ListNode *next;  
6:   *   ListNode(int x) : val(x), next(NULL) {}  
7:   * };  
8:   */  
9:  class Solution {  
10:  public:  
11:    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {  
12:      ListNode *dummy = new ListNode(-1);  
13:      ListNode *cur = dummy;  
14:      while (l1 && l2) {  
15:        if (l1->val < l2->val) { cur->next = l1; l1 = l1->next; }  
16:        else { cur->next = l2; l2 = l2->next; }  
17:        cur = cur->next;  
18:      }  
19:      if (l1) cur->next = l1;  
20:      if (l2) cur->next = l2;  
21:      cur = dummy->next;  
22:      delete dummy;  
23:      return cur;  
24:    }  
25:  };