Given a linked list, remove the nth node from the end of list and return its head.
** Notice**
The minimum number of nodes in list is n.

Example
Given linked list: 1->2->3->4->5->null, and n = 2.
After removing the second node from the end, the linked list becomes 1->2->3->5->null.

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @param n: An integer.
     * @return: The head of linked list.
     */
    ListNode removeNthFromEnd(ListNode head, int n) {
        // write your code here
        
        if (head == null) return null;
        
        ListNode p1 = head;
        ListNode p2 = head;
        ListNode prev = null;
        int i = 0;
        
        while (i < n) {
            p1 = p1.next;
            i++;
        }
        
        while (p1 != null) {
            prev = p2;
            p2 = p2.next;
            p1 = p1.next;
        }
        
        if (prev == null) {
            head = head.next;
            return head;
        } 
        
        prev.next = p2.next;
        return head;
        
    }
}

/*

public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        if (n <= 0) {
            return null;
        }
        // add one more to the list, then when the while loop ends, get the
        // preDelete node right away
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        
        ListNode preDelete = dummy;
        for (int i = 0; i < n; i++) {
            if (head == null) {
                return null;
            }
            head = head.next;
        }
        while (head != null) {
            head = head.next;
            preDelete = preDelete.next;
        }
        preDelete.next = preDelete.next.next;
        return dummy.next;
    }
}

*/