【leetcode】21. Merge Two Sorted Lists

@requires_authorization
@author johnsondu
@create_time 2015.7.14 10:35
@url [merge two sorted lists](https://leetcode.com/problems/merge-two-sorted-lists/)
/*******************
 * 按照大小直接进行归并
 * 时间复杂度：O(n)
 * 空间复杂度：O(n)
 ******************/
/**
 * 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) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        ListNode *head = new ListNode(0);
        ListNode *ptr = head;
        while(l1 || l2){
            ListNode *cur = NULL;
            if(l1 == NULL){
                cur = l2;
                l2 = l2->next;
            }
            else if(l2 == NULL){
                cur = l1;
                l1 = l1->next;
            }
            else{
                if(l1->val > l2->val){
                    cur = l2;
                    l2 = l2->next;
                }
                else{
                    cur = l1;
                    l1 = l1->next;
                }
            }
            ptr->next = cur;
            ptr = ptr->next;
        }

        return head->next;
    }
};