【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;
}
};