Leetcode_insertion-sort-list

地址：http://oj.leetcode.com/problems/insertion-sort-list/

Sort a linked list using insertion sort.

思路：模拟题，新建一个虚拟节点指向头节点，用节点tail维护已排序好的链表。

参考代码：

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *insertionSortList(ListNode *head) {
if(!head)
return head;
ListNode* virtual_head = new ListNode(-1<<31), *p = head->next, *cmp = NULL, *nxt = NULL, *tail = head;
virtual_head->next = head;
while(p)
{
nxt = p->next;
cmp = virtual_head;
while(cmp!=tail && cmp->next->val < p->val)
cmp = cmp->next;
if(cmp != tail)
{
p->next = cmp->next;
cmp->next = p;
}
else
{
tail = tail->next = p;
}
p = nxt;
}
tail->next = NULL;
return virtual_head->next;
}
};

不小心写了个选择排序的

class Solution {
private:
ListNode* merge(ListNode*l1, ListNode*l2) {
if(!l1)
return l2;
else if(!l2)
return l1;
ListNode*head = NULL, *p1 = l1, *p2 = l2, *cur = NULL;
while(p1 && p2) {
if(p1->val <= p2->val) {
if(!head)
head = cur = p1;
else
cur = cur->next = p1;
p1 = p1->next;
} else {
if(!head)
head = cur = p2;
else
cur = cur->next = p2;
p2 = p2->next;
}
}
if(p1)
cur->next = p1;
else
cur->next = p2;
return head;
}
void split(ListNode*head, ListNode**l1, ListNode**l2) {
ListNode* lnode = *l1, *rnode = *l2;
if(!head)
lnode = rnode = NULL;
else if(!head->next) {
lnode = head;
rnode = NULL;
} else {
lnode = rnode = head;
ListNode *tmp = NULL;
while(lnode) {
lnode = lnode->next;
if(lnode) {
lnode = lnode->next;
tmp = rnode;
rnode = rnode->next;
}
}
tmp->next = NULL;
*l1 = lnode = head;
*l2 = rnode;
}
}
public:
ListNode *sortList(ListNode *head) {
if(!head || !head->next)
return head;
ListNode*l1 = NULL, *l2 = NULL;
split(head, &l1, &l2);
l1 = sortList(l1);
l2 = sortList(l2);
return merge(l1, l2);
}
};