148. Sort List

看到时间复杂度是O（n log n)时，要想到用二分查找、归并排序、快速排序等算法。

public class Solution {
 
  public ListNode sortList(ListNode head) {
    if (head == null || head.next == null)
      return head;
       
    // step 1. cut the list to two halves
    ListNode prev = null, slow = head, fast = head;
   
    while (fast != null && fast.next != null) { prev = slow; slow = slow.next;
      fast = fast.next.next;
    }
   
    prev.next = null;
   
    // step 2. sort each half
    ListNode l1 = sortList(head);
    ListNode l2 = sortList(slow);
   
    // step 3. merge l1 and l2
    return merge(l1, l2);
  }
 
  ListNode merge(ListNode l1, ListNode l2) {
    ListNode l = new ListNode(0), p = l;
   
    while (l1 != null && l2 != null) {
      if (l1.val < l2.val) {
        p.next = l1;
        l1 = l1.next;
      } else {
        p.next = l2;
        l2 = l2.next;
      }
      p = p.next;
    }
   
    if (l1 != null)
      p.next = l1;
   
    if (l2 != null)
      p.next = l2;
   
    return l.next;
  }

}