[LeetCode]143 Reorder List

/**
* Definition for singly-linked list.
* class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) {
*         val = x;
*         next = null;
*     }
* }
*/
public class Solution {
public void reorderList(ListNode head) {

// Solution A:
reorderList_NoExtraSpace(head);

// Solution B:
// reorderList_Map(head);
}

//////////////////////////
// Solution A: NoExtraSpace
//
public void reorderList_NoExtraSpace(ListNode head)
{
// Hit to the middle
//
// Iterate list with 2 pointers.
// p1 with 1 pace
// p2 with 2 paces.
// Thus, when p2 reach end.
// p1 = size / 2 (odd)
//    = (size / 2) - 1 (even)

if (head == null || head.next == null)
return;

ListNode p1 = head;
ListNode p2 = head;
ListNode lastp1 = null;
while (p1 != null && p2 != null)
{
lastp1 = p1;
p1 = p1.next;
p2 = p2.next;
if (p2 == null)
p2 = null;
else
p2 = p2.next;
}
p2 = p1; //.next;
lastp1.next = null;
p1 = head;// 0, 1, 2, ...
p2 = reverse(p2); // n-1, n-2, n-3, ...

// Merge head & p1
while (p1 != null)
{
ListNode p1next = p1.next;
p1.next = p2;
ListNode p2next = null;
if (p2 != null)
{
p2next = p2.next;
p2.next = p1next;
p2 = p2next;
}
p1 = p1next;
}
}

private ListNode reverse(ListNode head)
{
ListNode tail = head;

if (tail == null)
return tail;

ListNode temp = head.next;
while (temp != null)
{
// Move tail to head.
// Temp tail.pre become new tail.
// go next

ListNode next = temp.next;
tail.next = next;
temp.next = head;
head = temp;
temp = next;
}
return head;
}

//////////////////////////
// Solution A: Map
//
// or using stack might be better
public void reorderList_Map(ListNode head)
{
if (head == null)
return;

Map<Integer, ListNode> map = new HashMap<>();

int size = 0;
ListNode temp = head;
while (temp != null)
{
map.put(size, temp);
size++;
temp = temp.next;
}

// Restruct
ListNode lastNode = null;
for (int i = 0 ; i < size ; i ++)
{
ListNode curNode = null;
if (i % 2 == 0)
{
curNode = map.get(i / 2);
}
else
{
curNode = map.get(size - 1 - i / 2);
}
if (lastNode != null)
lastNode.next = curNode;
curNode.next = null;
lastNode = curNode;
}
}
}