LeetCode 21 合并两个有序链表

我的

/**
 * 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* res;
        if(l1==NULL){
            res=l2;
            return res;
        }
        else if(l2==NULL){
            res=l1;
            return res;
        }
        else if((l2->val)<(l1->val)){
            res=l2;
            
            l2=l2->next;
        }
        else{
            res=l1;
            
            l1=l1->next;
        }
        ListNode* ans=res;
        while(1){
            if(l1==NULL){
                res->next=l2;
                return ans;
            }
            else if(l2==NULL){
                res->next=l1;
                return ans;
            }
            else if((l2->val)<(l1->val)){
                res->next=l2;
                res=res->next;
                l2=l2->next;
            }
            else{
                res->next=l1;
                res=res->next;
                l1=l1->next;
            }
        }
        
        return ans;
    }
};

 最快的：

/**
 * 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 preHead(0), *p = &preHead;
        while(l1 != NULL && l2 != NULL){
            if(l1->val <= l2->val){
                p->next = l1;
                l1 = l1->next;
            }else{
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
/*
        while(l1 != NULL){
            p->next = l1;
            l1 = l1->next;
            p = p->next;
        }
        while(l2 != NULL){
            p->next = l2;
            l2 = l2->next;
            p = p->next;
        }
*/
        if(l1 != NULL){
            p->next = l1;
            
        }
        if(l2 != NULL){
            p->next = l2;
            
        }
        return preHead.next;
    }
};

原来是注释里的，我改成了if。和我写的一样快。都是12ms。但是原提交却是8ms……

我写得长，因为res->为NULL，没想到可以return  preHead.next，学到了。