Stack vs Queue — What's the Difference?

A stack is last-in, first-out (LIFO) like a pile of plates; a queue is first-in, first-out (FIFO) like a line at a shop. They differ in which end you remove from.

The concept

Stack operations: push adds to the top, pop removes the top. It models undo history, call stacks and backtracking.

Queue operations: enqueue adds to the back, dequeue removes from the front. It models task scheduling, buffering and breadth-first search.

Add 1, 2, 3 then remove one

Practical examples

Stack with an array

const stack = []
stack.push(1); stack.push(2)
stack.pop() // 2

Queue (use a real deque for speed)

const queue = []
queue.push(1); queue.push(2)
queue.shift() // 1

Balanced brackets with a stack

function ok(s) {
  const st = [], m = { ')': '(', ']': '[' }
  for (const c of s) {
    if ('([').includes(c)) st.push(c)
    else if (m[c] && st.pop() !== m[c]) return false
  }
  return st.length === 0
}

Common mistakes

• Using array.shift() on huge arrays as a queue (O(n)).
• Mixing up which end gets removed.
• Forgetting to check for empty before pop/dequeue.

Key takeaways

• Stack = LIFO, queue = FIFO.
• Stacks for undo, recursion, parsing; queues for scheduling and BFS.
• Both give O(1) add and remove with the right structure.

FAQ

What is a deque?

A double-ended queue allowing add and remove at both ends.

What is a priority queue?

A queue that returns the highest-priority item first, often built on a heap.