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. http://leetcode.com/onlinejudge#question_21

leetcode_merge_two_sorted_list.cpp

/**
 * 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* ret = NULL;
        ListNode* cur = NULL;
        while (l1 != NULL || l2 != NULL) {
            int val = 0;
            if (l1 == NULL || (l2 != NULL && l2->val < l1->val)) {
                val = l2->val;
                l2 = l2->next;
            }
            else {
                val = l1->val;
                l1 = l1->next;
            }
            ListNode* node = new ListNode(val);
            if (cur == NULL) 
                ret = node;
            else
                cur->next = node;
            cur = node;
        }
        return ret;
    }
};