148.sort-list.cpp

/*
 * @lc app=leetcode id=148 lang=cpp
 *
 * [148] Sort List
 *
 * https://leetcode.com/problems/sort-list/description/
 *
 * algorithms
 * Medium (45.67%)
 * Likes:    3943
 * Dislikes: 169
 * Total Accepted:    341.4K
 * Total Submissions: 736K
 * Testcase Example:  '[4,2,1,3]'
 *
 * Given the head of a linked list, return the list after sorting it in
 * ascending order.
 * 
 * Follow up: Can you sort the linked list in O(n logn) time and O(1) memory
 * (i.e. constant space)?
 * 
 * 
 * Example 1:
 * 
 * 
 * Input: head = [4,2,1,3]
 * Output: [1,2,3,4]
 * 
 * 
 * Example 2:
 * 
 * 
 * Input: head = [-1,5,3,4,0]
 * Output: [-1,0,3,4,5]
 * 
 * 
 * Example 3:
 * 
 * 
 * Input: head = []
 * Output: []
 * 
 * 
 * 
 * Constraints:
 * 
 * 
 * The number of nodes in the list is in the range [0, 5 * 10^4].
 * -10^5 <= Node.val <= 10^5
 * 
 * 
 */

// @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) {
        if (!head || !head->next)
            return head;
        ListNode* mid = getMid(head);
        ListNode* left = sortList(head);
        ListNode* right = sortList(mid);
        return combine(left, right);
    }
    
    ListNode* getMid(ListNode* head) {
        ListNode* midPrev = nullptr;
        while (head && head->next) {
            midPrev = (midPrev == nullptr) ? head : midPrev->next;
            head = head->next->next;
        }
        ListNode* mid = midPrev->next;
        midPrev->next = nullptr;
        return mid;
    }
    
    ListNode* combine(ListNode* left, ListNode* right) {
        ListNode dummyHead(0);
        ListNode* ptr = &dummyHead;
        while (left && right) {
            if (left->val < right->val) {
                ptr->next = left;
                left = left->next;
            } else {
                ptr->next = right;
                right = right->next;
            }
            ptr = ptr->next;
        }
        if (left) ptr->next = left;
        else ptr->next = right;
        return dummyHead.next;
    }
};
// @lc code=end