021 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.

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#include <iostream>
#include <string>
#include <map>
#include <algorithm>
using namespace std;

struct ListNode
{
int val;
ListNode *next;
ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* head = NULL;
ListNode* l1pre = NULL;

if(l1==NULL) return l2;
else if (l2 == NULL) return l1;
else if(l2==NULL && l1==NULL) return NULL;

if(l1->val > l2->val) {
head = l2;
l2 = l1;
l1 = head;
}

head = l1;

for(; l1 != NULL && l2 != NULL;) {
if(l1->val <= l2->val) {
l1pre = l1;
l1 = l1->next;
} else {
ListNode *l2next = l2->next;
l2->next = l1;

if(l1pre!=NULL)
l1pre->next = l2;

l1pre = l2;
l2=l2next;
}
}

if(l1==NULL) {
l1pre->next = l2;
}
return head;
}
};