148. Sort List
采用归并排序,先将链表一分为二,对两部分进行排序,之后再合并两个排好序的链表。
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
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 dumy = new ListNode(0), p = dumy;
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 dumy.next;
}
}
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode sortList(ListNode head) {
if(head == null || head.next == null) {
return head;
}
ListNode pre = null;
ListNode slow = head;
ListNode fast = head;
while(fast != null && fast.next != null) {
pre = slow;
slow = slow.next;
fast = fast.next.next;
}
pre.next = null;
ListNode left = sortList(head);
ListNode right = sortList(slow);
// left and right are both sorted
return merge(left, right);
}
private ListNode merge(ListNode left, ListNode right) {
if(left == null) {
return right;
}
if(right == null) {
return left;
}
if(left.val < right.val) {
left.next = merge(left.next, right);
return left;
} else {
right.next = merge(left, right.next);
return right;
}
}
}