/* reverse head -> 1 -> 2 -> 3 -> 4 to head -> 4 -> 3 -> 2 -> 1
*
* reverse while traversal
*
* The reverse method traversals the linked list once,
* the time complexity is O(N).
*
* This solution preserves original linked list
* the space complexity is O(N).
*
*
* */
public class SolutionTwo {
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);
}
/*
* copy current node's next node value to a new node, have it point backwards
* */
public static MyNode reverse(MyNode head) {
if(head == null || head.next == null) return head;
//first node will become the last node
MyNode current = head.next;
//special treatment of the last node
MyNode newLink = new MyNode(current.value, null);
while(current.next != null) {
//create a new node with the current node's next node value, the new node should point backwards
newLink = new MyNode(current.next.value, newLink);
//move one step further
current = current.next;
}
//update head
MyNode newHead = new MyNode(0, newLink);
return newHead;
}
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;
}
}
