Sort a linked list in O(n log n) time using constant space complexity.
C++
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* sortList(ListNode* head){
if (!head||!head->next) return head;
ListNode* slow = head;
ListNode* fast = head;
ListNode* pre = head;
while (fast && fast->next){
pre = slow;
slow = slow->next;
fast = fast->next->next;
}
pre->next = NULL;
return merge(sortList(head),sortList(slow));
}
ListNode* merge(ListNode* left,ListNode* right){
ListNode *head;
if (left->val < right->val){
head = left;
left = left->next;
}else{
head = right;
right = right->next;
}
ListNode *cur = head;
while(left && right){
if (left->val < right->val){
cur->next = left;
left = left->next;
}else{
cur->next = right;
right = right->next;
}
cur = cur->next;
}
if (left) cur->next = left;
if (right) cur->next = right;
return head;
}
ListNode* merge2(ListNode*left, ListNode* right){
ListNode* head = new ListNode(-1);
ListNode* cur = head;
while(left && right){
if (left->val < right->val){
cur->next = left;
left = left->next;
}else{
cur->next = right;
right = right->next;
}
cur = cur->next;
}
if (left) cur->next = left;
if (right) cur->next = right;
return head->next;
}
};