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Sort List

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Description

###Quicksort

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/**
 * 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 == NULL || head->next == NULL) return head;
        sort(head, NULL);
        return head;
    }
    
    void sort(ListNode* start, ListNode* end) {
        if (start == end || start->next == end) return;
        ListNode* mid = partition(start, end);
        sort(start, mid);
        sort(mid->next, end);
    }
    
    ListNode* partition(ListNode* start, ListNode* end) {
        int midValue = start->val;
        ListNode* cur = start->next;
        ListNode* mid = start;
        while (cur != end) {
            if (cur->val < midValue) {
                mid = mid->next;
                swap(mid->val, cur->val);
            }
            cur = cur->next;
        }
        swap(mid->val, start->val);
        return mid;
    }
};

Mergesort

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/**
 * 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 == NULL || head->next == NULL) return head;
        ListNode* ret = sort(head);
        return ret;
    }
    
    ListNode* sort(ListNode* start) {
        if (!start || !start->next) return start;
        ListNode* slow = start;
        ListNode* quick = start;
        while (quick && quick->next && quick->next->next) {
            slow = slow->next;
            quick = quick->next->next;
        }
        ListNode* left = sort(slow->next);
        slow->next = NULL;
        ListNode* right = sort(start);
        return merge(left, right);
    }
    
    ListNode* merge(ListNode* left, ListNode* right) {
        ListNode* fake = new ListNode(-1);
        ListNode* cur = fake;
        while (left != NULL && right != NULL) {
            if (left->val < right->val){
                cur->next = left;
                left = left->next;
            }else {
                cur->next = right;
                right = right->next;
            }
            cur = cur->next;
        }
        cur->next = left ? left : right;
        ListNode* ret = fake->next;
        delete fake;
        return ret;
    }
};