合并两个有序列表

/**
 * 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 result = ListNode(-1);
        ListNode* prev = &result;
        while(l1 && l2) {
            if(l1->val <= l2->val) {
                prev->next = l1;
                l1 = l1->next; 
            } else {
                prev->next = l2;
                l2 = l2->next;
            }
            prev = prev->next;            
        }            
    prev->next = l1? l1:l2;
    return result.next;
    }
    
};