LeetCode: Sort List

merge sort

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *mergesort(ListNode *p, ListNode *q) {
        ListNode *head = NULL;
        ListNode *cur = NULL;
        while (p && q) {
            if (p->val < q->val) {
                if (!head) head = cur = p;
                else {
                    cur->next = p;
                    cur = cur->next;
                }
                p = p->next;
            }
            else {
                if (!head) head = cur = q;
                else {
                    cur->next = q;
                    cur = cur->next;
                }
                q = q->next;
            }
        }
        if (p) cur->next = p;
        else cur->next = q;
        return head;
    }
    ListNode *sortList(ListNode *head) {
        if (!head || !head->next) return head;
        ListNode *p, *q;
        p = q = head;
        while (q && q->next) {
            q = q->next->next;
            if (!q) break;
            p = p->next;
        }
        q = p->next;
        p->next = NULL;
        p = sortList(head);
        q = sortList(q);
        return mergesort(p, q);
    }
};

 C#

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode SortList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode p = head, q = head;
        while (q != null && q.next != null) {
            q = q.next.next;
            if (q == null) break;
            p = p.next;
        }
        q = p.next;
        p.next = null;
        p = SortList(head);
        q = SortList(q);
        return mergesort(p, q);
    }
    public ListNode mergesort(ListNode p, ListNode q) {
        ListNode head = null;
        ListNode cur = null;
        while (p != null && q != null) {
            if (p.val < q.val) {
                if (head == null) head = cur = p;
                else {
                    cur.next = p;
                    cur = cur.next;
                }
                p = p.next;
            }
            else {
                if (head == null) head = cur = q;
                else {
                    cur.next = q;
                    cur = cur.next;
                }
                q = q.next;
            }
        }
        if (p != null) cur.next = p;
        else cur.next = q;
        return head;
    }
}