链表的归并排序与快速排序

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

class Solution {
public:
ListNode* sortList(ListNode* head) {
if (head == nullptr || head->next == nullptr)
return head;
ListNode *fast = head, *slow = head;
while (fast->next && fast->next->next)
{
slow = slow->next;
fast = fast->next->next;
}

fast = slow->next;
slow->next = nullptr;
head = sortList(head);
fast = sortList(fast);
return merge(head, fast);
}

private:
ListNode* merge(ListNode *l1, ListNode *l2)
{
ListNode empty(-1);
ListNode *tail = ∅
ListNode *tmp;
for (; l1 || l2; tail = tail->next)
{
if (l1 == nullptr)
{
tail->next = l2;
break;
}
if (l2 == nullptr)
{
tail->next = l1;
break;
}
if (l1->val <= l2->val)
{
tmp = l1;
l1 = l1->next;
}
else
{
tmp = l2;
l2 = l2->next;
}
tail->next = tmp;
}

return empty.next;
}
};

class Solution {
public:
ListNode* sortList(ListNode* head) {
return sort(head, nullptr);
}
private:
void swap(ListNode *a, ListNode *b)
{
int tmp = a->val;
a->val = b->val;
b->val = tmp;
}
ListNode *sort(ListNode *begin, ListNode *end)
{
if (begin == nullptr || begin == end || begin->next == end)
return begin;
int key = begin->val;
ListNode *p = begin;
ListNode *q = begin->next;
for (; q != end; q = q->next)
{
if (q->val < key)
{
p = p->next;
swap(p, q);
}
}
swap(p, begin);
sort(begin, p);
sort(p->next, end);

return begin;
}
};

class Solution {
public:
ListNode* sortList(ListNode* head) {
return sort(head, nullptr);
}
private:
ListNode *sort(ListNode *begin, ListNode *end)
{
if (begin == nullptr || begin == end || begin->next == end)
return begin;
ListNode *part = patition(begin, end);

ListNode *first = sort(begin, part);
ListNode *second = sort(part->next, end);
part->next = second;

return first;
}

ListNode *patition(ListNode * &begin, ListNode *end)
{
ListNode *head = begin;
ListNode *tail = begin;
ListNode *cur = begin->next;
while (cur != end)
{
if (cur->val < begin->val)
{
tail->next = cur->next;
cur->next = head;
head = cur;
cur = tail;
}

tail = cur;
cur = cur->next;
}
ListNode *part = begin;
begin = head;
return part;
}
};