Merge Two Sorted Lists

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.

思路：直接合并，代码如下：

/**
* 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;
if(l1->val > l2->val)//l1的首个元素保持最小
swap(l1, l2);
ListNode *p = l1, *q = l2, *pre;
while(p && q){
if(p->val <= q->val){
pre = p;
p = p->next;
}
else{
ListNode *tmp = new ListNode(q->val);
tmp->next = pre->next;
pre->next = tmp;
pre = pre->next;
q = q->next;
}
}
if(q)
pre->next = q;
return l1;
}
};