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21. Merge two sorted linked lists
Merges two ascending lists into a new ascending list and returns. The new linked list is formed by splicing all the nodes of the given two linked lists.
Link:
21. Merge two ordered linked listslink
Method 1: Take the small tail plug
1.1 Code:
struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2)
{
struct ListNode* head=NULL;
struct ListNode* tail=NULL;
if(list1==NULL)//判断是否是空链表
return list2;
if(list2==NULL)//判断是否是空链表
return list1;
while(list1&&list2)
{
if(list1->val<list2->val)
{
if(tail==NULL)
{
head=tail=list1;
}
else
{
tail->next=list1;
tail=tail->next;
}
list1=list1->next;
}
else
{
if(tail==NULL)
{
head=tail=list2;
}
else
{
tail->next=list2;
tail=tail->next;
}
list2=list2->next;
}
}
if(list1)
{
tail->next=list1;
}
if(list2)
{
tail->next=list2;
}
return head;
}
1.2 Flowchart:
1.3 Note:
- Pay attention to judging whether list1 and ;ist2 are empty linked lists
- To consider the case of the first insertion is no node
Method 2: with sentinel position
The so-called linked list with a sentinel bit is an additional linked list node, which is the first node,its value field does not store anything, are referenced only for operational convenience.
If a linked list has sentinel nodes, then the first element of the linear list should be the second node of the linked list.
2.1 Code:
struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2)
{
struct ListNode* head=NULL;
struct ListNode* tail=NULL;
head=tail=(struct ListNode*)malloc(sizeof(struct ListNode));
if(list1==NULL)
return list2;
if(list2==NULL)
return list1;
while(list1&&list2)
{
if(list1->val<list2->val)
{
tail->next=list1;
tail=tail->next;
list1=list1->next;
}
else
{
tail->next=list2;
tail=tail->next;
list2=list2->next;
}
}
if(list1)
{
tail->next=list1;
}
if(list2)
{
tail->next=list2;
}
struct ListNode* del=head;
head=head->next;
free(del);
del=NULL;
return head;
}
2.2 Flowchart:
Summarize
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