Linked List Two Pointer Problems

Written by Swapnil RajaPublished 28 Aug 2026

You cannot index into a linked list, so every question about position is answered with two pointers moving at different speeds. Learn the three shapes and you have covered most of what gets asked.

Why speed differences work

If one pointer moves twice as fast as another, then when the fast one reaches the end, the slow one is exactly halfway. Every trick here is a variation on that.

Find the middle

Fast and slow

javascript

function middle(head) {
  let slow = head
  let fast = head

  while (fast && fast.next) {
    slow = slow.next
    fast = fast.next.next
  }

  return slow
}

PreviousLinked Lists Explained NextRecursion Explained

Floyd's cycle detection

javascript

function hasCycle(head) {
  let slow = head
  let fast = head

  while (fast && fast.next) {
    slow = slow.next
    fast = fast.next.next
    if (slow === fast) return true
  }

  return false
}

Gap of n

javascript

function nthFromEnd(head, n) {
  let lead = head

  for (let i = 0; i < n; i++) {
    if (!lead) return null   // list shorter than n
    lead = lead.next
  }

  let trail = head
  while (lead) {
    lead = lead.next
    trail = trail.next
  }

  return trail
}

Removing without special cases

javascript

function removeValue(head, target) {
  const dummy = new Node(null, head)
  let current = dummy

  while (current.next) {
    if (current.next.value === target) {
      current.next = current.next.next   // skip it
    } else {
      current = current.next
    }
  }

  return dummy.next   // may differ from the original head
}

Linked List Two Pointer Problems

Why speed differences work

Find the middle

Detect a cycle

Nth node from the end

The dummy head trick