Leetcode 21 Merge Two Sorted Lists 链表

/**
* 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 && !l2) return l1;
if(l2 && !l1) return l2;
if( !l2 && !l1) return NULL; //保证所有列表不为空
ListNode* t1 = l1;
ListNode* t2 = l2;
ListNode* t = NULL;
if(t1->val < t2->val){  //确定表头t是l1还是l2
t = t1;
t1 = t1->next;
}
else{
t = t2;
t2 = t2->next;
}
for(;t1&&t2; t = t->next){//确定表头t的下一个元素
if(t1->val < t2->val){
t->next = t1;
t1 = t1->next;
}
else{
t->next = t2;
t2 = t2->next;
}
}
if(t1){//t1不为空，将l1剩余部分插入到t后
t->next = t1;
}
if(t2){//t2不为空，将l2剩余部分插入到t后
t->next = t2;
}
if(l1->val < l2->val) return l1;
else return l2;//确定表头
}
};