归并排序链表

#include <iostream>
using namespace std;

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

ListNode *merge(ListNode *head1, ListNode *head2){
if (head1 == NULL)
return head2;
if (head2 == NULL)
return head1;

ListNode *head;
if (head1->val < head2->val){
head = head1;
head1 = head1->next;
}
else{
head = head2;
head2 = head2->next;
}

//两个链表不为空
ListNode *p = head;
while(head1 != NULL && head2 != NULL){
if (head1->val < head2->val){
p->next = head1;
head1 = head1->next;
}
else{
p->next = head2;
head2 = head2->next;
}
p = p->next;
}

//一个链表不为空
if (head1 != NULL){
p->next = head1;
}
else if (head2 != NULL){
p->next = head2;
}

return head;
}

ListNode *merge_sort_list(ListNode *head){
if (head == NULL || head->next == NULL){  //0个或1个结点
return head;
}

//找到中间结点
ListNode *fast = head;
ListNode *slow = head;
while(fast->next != NULL && fast->next->next != NULL){
fast = fast->next->next;
slow = slow->next;
}
ListNode *fast_start = slow->next;
slow->next = NULL;

fast = merge_sort_list(fast_start); //后半段
slow = merge_sort_list(head); //前半段
return merge(fast, slow);
}

int main(){
ListNode *head = new ListNode(0);
ListNode *node1 = new ListNode(3);
ListNode *node2 = new ListNode(1);
ListNode *node3 = new ListNode(2);
ListNode *node4 = new ListNode(6);
ListNode *node5 = new ListNode(5);
ListNode *node6 = new ListNode(4);

head->next = node1;
node1->next = node2;
node2->next = node3;
node3->next = node4;
node4->next = node5;
node5->next = node6;
ListNode *p = head;
while(p){
cout << p->val << '\t';
p = p->next;
}
cout << endl;

ListNode *result = merge_sort_list(head);

p = result;
while(p){
cout << p->val << '\t';
p = p->next;
}
cout << endl;

int ttt = 0;
return 0;
}