21.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* head = new ListNode(0);
        ListNode* curr = head;
        
        while (l1 != NULL && l2 != NULL) {
            if (l1 -> val <= l2 -> val) {
                curr -> next = l1;
                l1 = l1 -> next;
            }
            else {
                curr -> next = l2;
                l2 = l2 -> next;
            }
            curr = curr -> next;
        }
        
        if (l1 == NULL) {
            curr -> next = l2;
        }
        else {
            curr -> next = l1;
        }
        
        return head -> next;
    }
};