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public class SortList {
public ListNode sortList(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode fast = head;
ListNode slow = head;
while (fast.next != null && fast.next.next != null) {
fast = fast.next.next;
slow = slow.next;
}
fast = slow;
slow = slow.next;
fast.next = null;
fast = sortList(head);
slow = sortList(slow);
return merge(fast, slow);
}
private ListNode merge(ListNode a, ListNode b) {
ListNode result;
if (a == null) {
return b;
}
if (b == null) {
return a;
}
if (a.val >= b.val) {
result = b;
b = b.next;
} else {
result = a;
a = a.next;
}
ListNode cursor = result;
while (a != null && b != null) {
if (a.val >= b.val) {
cursor.next = b;
b = b.next;
} else {
cursor.next = a;
a = a.next;
}
cursor = cursor.next;
}
cursor.next = a == null ? b : a;
return result;
}
@Test
public void test() {
ListNode head = new ListNode(1);
head.next = new ListNode(2);
sortList(head);
}
}
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