19. Remove Nth Node From End of List

Given a linked list, remove the nth node from the end of list and return its head.

For example,

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

Note:
Given n will always be valid.
Try to do this in one pass.

 

public ListNode removeNthFromEnd(ListNode head, int n) {
    ListNode dummy = new ListNode(-1);
    ListNode slow = dummy;
    ListNode fast = dummy;
    dummy.next = head;

    //Move fast in front so that the gap between slow and fast becomes n
    for(int i=0; i<=n; i++)   {
        if (fast == null) {
            return null;
        }
        fast = fast.next;
    }
    //Move fast to the end, maintaining the gap
    while(fast != null) {
        slow = slow.next;
        fast = fast.next;
    }
    //Skip the desired node
    slow.next = slow.next.next;
    return dummy.next;
}