148. Sort List

Sort a linked list in O(n log n) time using constant space complexity.

 

//利用归并排序的思想

public ListNode sortList(ListNode head) {
  if (head == null || head.next == null)
    return head;
  ListNode prev = null;
  ListNode slow = head;
  ListNode fast = head;
  while (fast != null && fast.next != null ) {
    prev = slow;
    slow = slow.next;
    fast = fast.next.next;
  }

  prev.next = null;//将前半段链表设置为null结尾，链表被分成(head,p),(slow,fast)

  ListNode l1 = sortList(head);
  ListNode l2 = sortList(slow);

  return merge(l1, l2);
}

private ListNode merge(ListNode l1, ListNode l2) {
  // 见合并两个排序的链表
}