/* reverse head -> 1 -> 2 -> 3 -> 4 to head -> 4 -> 3 -> 2 -> 1
*
* in place reverse
*
* step 1. change the node 1's next to null
* step 2. change node 2's next to node 1
* step 3. change node 3's next to node 2
* ...
* finally. change head's next to node 4
*
* The reverse method traversals the linked list once,
* the time complexity is O(N).
*
* This solution didn't create a new linked list, it changes the node's next value in place,
* the space complexity is O(1).
*
* */
public class SolutionFour {
public static MyNode reverse(MyNode head) {
if(head == null || head.next == null) return head;
MyNode pre = head.next;
MyNode current = pre.next;
MyNode next = null;
//set the tail node
pre.next = null;
//have the current node point to previous node
while(current != null) {
next = current.next;
current.next = pre;
pre = current;
current = next;
}
//have the head point to the last node of the original linked list
head.next = pre;
return head;
}
public static void main(String...args) {
MyNode head = createLinkedList();
MyNode preHead = new MyNode(0, head.next);
printResult(head);
head = reverse(head);
printResult(head);
printResult(preHead);
}
public static void printResult(MyNode head) {
if(head == null) return;
MyNode current = head;
System.out.println();
while (current.next != null) {
current = current.next;
System.out.print(current.value + " ");
}
}
public static MyNode createLinkedList() {
MyNode head = new MyNode(0, null);
MyNode next = new MyNode(1, null);
head.next = next;
next.next = new MyNode(2, null);
next = next.next;
next.next = new MyNode(3, null);
next = next.next;
next.next = new MyNode(4, null);
next = next.next;
next.next = new MyNode(5, null);
next = next.next;
next.next = new MyNode(6, null);
next = next.next;
return head;
}
}
