Slow Fast

We take two pointers and move them at different speeds. Examples:

Python

def getMiddle(head):
    if not head:
        return None
    slow, fast = head, head
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next
    return slow.val

# n'th node from the end
def nthNodeFromEnd(head, n):
    if not head:
        return -1
    fast = head
    for _ in range(n):
        if not fast:
            return -1
        fast = fast.next
    slow = head
    while fast:
        slow = slow.next
        fast = fast.next
    return slow.val