/
148.sort-list.cpp
90 lines (83 loc) · 2.22 KB
/
148.sort-list.cpp
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/*
* @lc app=leetcode id=148 lang=cpp
*
* [148] Sort List
*/
// @lc code=start
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode() : val(0), next(nullptr) {}
* ListNode(int x) : val(x), next(nullptr) {}
* ListNode(int x, ListNode *next) : val(x), next(next) {}
* };
*/
class Solution
{
public:
ListNode *sortList(ListNode *head)
{
// only 0/1 element in the linked lists
if (!head || !head->next)
return head;
//use fast-slow ptrs to find mid
ListNode *fast = head;
ListNode *slow = head;
while (fast->next && fast->next->next)
{
fast = fast->next->next;
slow = slow->next;
} // while
// break the list
ListNode *slowNext = slow->next;
slow->next = nullptr;
ListNode *left = sortList(head);
ListNode *right = sortList(slowNext);
return merge(left, right);
}
//* huahua's version of merge is better
ListNode *merge(ListNode *left, ListNode *right)
{
ListNode dummy(0);
ListNode *dummy_ptr = &dummy;
while (left && right)
{
if (left->val < right->val)
{
ListNode *next = left->next;
left->next = nullptr;
dummy_ptr->next = left;
dummy_ptr = dummy_ptr->next;
left = next;
}
else
{
ListNode *next = right->next;
right->next = nullptr;
dummy_ptr->next = right;
dummy_ptr = dummy_ptr->next;
right = next;
} // end if
} // while
while (right)
{
ListNode *next = right->next;
right->next = nullptr;
dummy_ptr->next = right;
dummy_ptr = dummy_ptr->next;
right = next;
} // while
while (left)
{
ListNode *next = left->next;
left->next = nullptr;
dummy_ptr->next = left;
dummy_ptr = dummy_ptr->next;
left = next;
} //while
return dummy.next;
}
};
// @lc code=end