leetcode — reorder-list

/**
* Source : https://oj.leetcode.com/problems/reorder-list/ *
* Given a singly linked list L: L0→L1→…→Ln-1→Ln,
* reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
*
* You must do this in-place without altering the nodes' values.
*
* For example,
* Given {1,2,3,4}, reorder it to {1,4,2,3}.
*/
public class RecordList {

/**
* 将链表的后半部分翻转之后依次插入链表前半部分每个元素后面
*
* 设链表长度为n
* 1. 找到链表后半部分起始位置： n/2+1，使用双指针法，slow每次移动一个，fast每次移动两个，fast移动到最后的时候，slow指向的正好是 n/2+1
* 2. 反转后半部分连链表
* 3. 将反转后的后半部分链表依次插入前半部分,left指向左边，right指向反转后的第一个node，依次插入left的后一个，直到right指向null，
*      right每次移动一个node，left每次移动2个node（因为刚刚left后面插入一个节点）
*
* @param head
* @return
*/
public LinkedNode record (LinkedNode head) {
if (head == null || head.next == null) {
return head;
}
LinkedNode midNode = findMidNode(head);
LinkedNode reversedList = reverse(midNode);
LinkedNode left = head;
LinkedNode right = reversedList;

while (right != null && right.next != null) {
// 记录将要被插入的元素
LinkedNode target = right;
// 下一个需要被插入的元素
right = right.next;
// 插入target到链表中
target.next = left.next;
left.next = target;
left = left.next.next;
}

return head;
}

private LinkedNode findMidNode (LinkedNode head) {
LinkedNode slow = head;
LinkedNode fast = head;
while (fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
}
return slow;
}

private LinkedNode reverse (LinkedNode head) {
LinkedNode p = head;
LinkedNode pre = null;
while (p != null) {
LinkedNode next = p.next;
p.next = pre;
pre = p;
p = next;
}
return pre;
}

private class LinkedNode {
int value;
LinkedNode next;

}

/**
* 创建普通的链表
* @param arr
* @return
*/
public LinkedNode createList (int[] arr) {
if (arr.length == 0) {
return null;
}
LinkedNode head = new LinkedNode();
head.value = arr[0];
LinkedNode pointer = head;
for (int i = 1; i < arr.length; i++) {
LinkedNode node = new LinkedNode();
node.value = arr[i];
pointer.next = node;
pointer = pointer.next;
}
return head;
}

private static void print (LinkedNode head) {
if (head == null) {
System.out.println("[]");
}
StringBuffer stringBuffer = new StringBuffer("[");
while (head != null) {
stringBuffer.append(head.value);
stringBuffer.append(",");
head = head.next;
}
stringBuffer.deleteCharAt(stringBuffer.length()-1);
stringBuffer.append("]");
System.out.println(stringBuffer);
}

public static void main(String[] args) {
RecordList recordList = new RecordList();
int[] arr = new int[]{1,2,3,4,5,6};
print(recordList.record(recordList.createList(arr)));

}
}