My code:
/**
* 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)
return null;
int count = 0;
ListNode temp = head;
while (temp != null) {
count++;
temp = temp.next;
}
return sortList(head, count);
}
private ListNode sortList(ListNode head, int count) {
if (count <= 1)
return head;
int left = 0;
int right = 0;
if (count % 2 == 0)
left = count / 2;
else
left = count / 2 + 1;
right = count - left;
ListNode leftHead = head;
ListNode rightHead = null;
int countNode = 0;
ListNode temp = head;
while (temp != null) {
countNode++;
if (countNode == left) {
rightHead = temp.next;
temp.next = null;
break;
}
else
temp = temp.next;
}
ListNode subLeftHead = sortList(leftHead, left);
ListNode subRightHead = sortList(rightHead, right);
ListNode dummyNode = new ListNode(-1);
ListNode tempDummy = dummyNode;
int countLeft = 0;
int countRight = 0;
while (countLeft < left && countRight < right) {
if (subLeftHead.val <= subRightHead.val) {
tempDummy.next = subLeftHead;
subLeftHead = subLeftHead.next;
tempDummy = tempDummy.next;
countLeft++;
}
else {
tempDummy.next = subRightHead;
subRightHead = subRightHead.next;
tempDummy = tempDummy.next;
countRight++;
}
}
if (countLeft == left)
tempDummy.next = subRightHead;
else
tempDummy.next = subLeftHead;
return dummyNode.next;
}
public static void main(String[] args) {
ListNode n1 = new ListNode(8);
ListNode n2 = new ListNode(7);
ListNode n3 = new ListNode(6);
ListNode n4 = new ListNode(5);
ListNode n5 = new ListNode(4);
ListNode n6 = new ListNode(3);
ListNode n7 = new ListNode(2);
ListNode n8 = new ListNode(1);
n1.next = n2;
n2.next = n3;
n3.next = n4;
n4.next = n5;
n5.next = n6;
n6.next = n7;
n7.next = n8;
Solution test = new Solution();
ListNode head = test.sortList(n1);
while (head != null) {
System.out.println(head.val);
head = head.next;
}
}
}
My test result:
Paste_Image.png
这次作业说实话不难，但是要完全写出来还是有点细节的。
首先，一开始我很难理解，链表麻痹怎么排序啊，还是归并排序。
后来发现必须得统计结点个数，接下来就好做点了。
用递归么，一层层递归下去，用普林斯顿算法课的说法叫做， bottom -up.
分成一块一块，然后再一块块拼装起来。
思想还是归并排序的思想，在这里我想说一个小技巧，那就是， dummy node.
真的很好用，尤其对于链表，很多复杂情况不需要在考虑了。
还有这里有个细节，
因为我们要把一个链表切成两段，然后分别递归，事实上这两个子链表还是连接在一起的，所以我们需要人为得把他们切断，
即 temp.next = null;
然后再递归。这是一个注意点。
同时，链表操作我还是有些地方没注意到，像这道题目，通过dummy node来将两个子链表合二为一，并且排序，没有用到额外的空间，dummy node 就像是细线一样，将这些结点重新串在了一块儿。
**
总结： Merge Sort, LinkedList, Recursion
刚写代码，一个很久以前的朋友突然找我，上来第一句话就是，
以后去哪里发展？
很有社会上的口气。我就和他聊了开来，一开始是有提防心的，比如他问我在不在家，之类的，我都回避，但聊着聊着就聊开了。某人估计又要骂我傻逼了吧。
他要结婚了，然后一直说我了不起，本科，研究生的大学都很强，学历高。
说真的，我从来没觉得我的学校很强，觉得可以拿这个去比，从来没觉得。
但是，从他嘴中，我了解到了社会，尤其中国社会，原来这么看重学历。甚至高中是哪里的都很看重。那我应该更加自信点了，虽然以前我一直觉得一个人该有相当的本事再有相当的自信，现在看来，学历也是那么重要，即使你什么都不会。
但是，我现在很努力，为什么？就像我之前努力出国，为什么，真的不是为了学历。。当然学校是否有名也是有考虑的，但更多的，是想有一个好的环境让我去学习技术。所以其实我的出发点还是比较纯洁的，这也可能是我为什么能持续走下去的原因之一吧，因为我真的有这个需求，我很想学习这方面的知识。
女朋友的事，我这几天是有点操之过急了，毕竟她才培训一个礼拜啊，那么忙，身体也那么累，不像我回家还休息了一两天，然后之后也是懒散的学习。我真的把她当成了机器，虽然我的本心是好的，但我就是把她当成了机器，按照我的设计而去学习。这是不对的。
但同样，她也还是不够拼命，也许还没开始吧。希望下周可以真正开始了。
Because TOEFL is on the way.
**
Anyway, Good luck, Richardo!
My code:
/**
* 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)
return null;
return sort(head);
}
/** sort this list */
private ListNode sort(ListNode head) {
if (head == null || head.next == null)
return head;
ListNode slow = head;
ListNode fast = head;
/** find the middle node in the linkedlist using slow and fast pointers */
while (fast.next != null && fast.next.next != null) {
slow = slow.next; // jump 1 node for slow pointer
fast = fast.next; // jump 2 nodes for fast pointer
fast = fast.next;
}
ListNode leftHead = head;
ListNode rightHead = slow.next;
slow.next = null; // cut the connection between left and right sub linked list
/** sort these two sub linked lists separately */
leftHead = sort(leftHead); // make sure this sub lis is sorted
rightHead = sort(rightHead);
/** merge them into one sorted linked list */
return merge(leftHead, rightHead);
}
/** merge these two sub sorted lists */
private ListNode merge(ListNode leftHead, ListNode rightHead) {
if (leftHead == rightHead)
return leftHead;
ListNode leftScanner = leftHead;
ListNode rightScanner = rightHead;
ListNode head;
/** ensure the head node */
if (leftHead.val < rightHead.val) {
head = leftHead;
leftScanner = leftScanner.next;
}
else {
head = rightHead;
rightScanner = rightScanner.next;
}
ListNode scanner = head;
/** merge two linked lists until one of them is finished */
while (leftScanner != null && rightScanner != null) {
if (leftScanner.val < rightScanner.val) {
scanner.next = leftScanner;
leftScanner = leftScanner.next;
}
else {
scanner.next = rightScanner;
rightScanner = rightScanner.next;
}
scanner = scanner.next;
}
/** connect the rest of the other list to this new sorted list */
if (leftScanner == null)
scanner.next = rightScanner;
else
scanner.next = leftScanner;
return head;
}
public static void main(String[] args) {
Solution test = new Solution();
ListNode a0 = new ListNode(3);
ListNode a1 = new ListNode(2);
ListNode a2 = new ListNode(4);
a0.next = a1;
a1.next = a2;
ListNode head = test.sortList(a0);
while (head != null) {
System.out.println(head.val);
head = head.next;
}
}
}
这道题目写了一会儿。以前的写法是不对的。因为我声明了一个dummy node需要内存，然后一层层下来就是， log(n) 的复杂度，就不再是常数级别了。
然后其他就差不多了，就是 merge sort the linked list
Anyway, Good luck, Richardo!
My code:
/**
* 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) {
return helper(head);
}
private ListNode helper(ListNode head) {
if (head == null || head.next == null) {
return head;
}
ListNode slow = head;
ListNode fast = head;
while (fast.next != null && fast.next.next != null) {
slow = slow.next;
fast = fast.next.next;
}
ListNode right = slow.next;
ListNode left = head;
slow.next = null;
left = helper(left);
right = helper(right);
return merge(left, right);
}
private ListNode merge(ListNode leftHead, ListNode rightHead) {
if (leftHead == null) {
return rightHead;
}
else if (rightHead == null) {
return leftHead;
}
ListNode left = leftHead;
ListNode right = rightHead;
ListNode dummy = new ListNode(-1);
ListNode curr = dummy;
while (left != null || right != null) {
if (left == null) {
curr.next = right;
curr = curr.next;
right = right.next;
}
else if (right == null) {
curr.next = left;
curr = curr.next;
left = left.next;
}
else if (left.val < right.val) {
curr.next = left;
curr = curr.next;
left = left.next;
}
else {
curr.next = right;
curr = curr.next;
right = right.next;
}
}
return dummy.next;
}
}
这道题目本身没有什么难度。空间复杂度是O(1),虽然用了dummy node,但是是在merge中使用的，使用完了后立刻释放。
Anyway, Good luck, Richardo! — 08/17/2016
