# Sort List

``````给出1-3->2->null，给它排序变成1->2->3->null
``````

merge sort

## Code

``````/**
* Definition for ListNode.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int val) {
*         this.val = val;
*         this.next = null;
*     }
* }
*/
public class Solution {
/**
* @param head: The head of linked list.
* @return: You should return the head of the sorted linked list,
using constant space complexity.
*/
private ListNode findMiddle(ListNode head) {
ListNode slow = head, fast = head.next;
while (fast != null && fast.next != null) {
fast = fast.next.next;
slow = slow.next;
}
return slow;
}

private ListNode merge(ListNode head1, ListNode head2) {
ListNode dummy = new ListNode(0);
ListNode tail = dummy;
while (head1 != null && head2 != null) {
if (head1.val < head2.val) {
tail.next = head1;
head1 = head1.next;
} else {
tail.next = head2;
head2 = head2.next;
}
tail = tail.next;
}
if (head1 != null) {
tail.next = head1;
} else {
tail.next = head2;
}

return dummy.next;
}

public ListNode sortList(ListNode head) {
if (head == null || head.next == null) {
return head;
}

ListNode mid = findMiddle(head);

ListNode right = sortList(mid.next);
mid.next = null;
ListNode left = sortList(head);

return merge(left, right);
}
}
``````