Stacks and Queues

Queue from Two Stacks

Implement queue behavior with an input stack and an output stack.

Algorithm

The replay uses the same three values in every language, so this Kotlin DSA implementation can be compared directly with the rest of the DSA track.

input stack Enqueue pushes new values onto the input stack.
output stack When the output stack is empty, transferring all input values reverses them into dequeue order.

Visual walkthrough

The two-stack queue keeps cheap enqueues in the input stack, then reverses that stack only when dequeue needs the output stack.

Step 1 - Enqueue pushes onto the input stack

After enqueueing 10, 20, 30, the newest input value is on top of the input stack.

After enqueues: input top is 30; output is empty.input stack top -> bottomoutput stack top -> bottomremoved30(empty)(empty)2010

Step 2 - Transfer reverses into output order

Moving every input value to the output stack turns 10 into the next pop.

After transfer: output top is 10, so dequeue returns the oldest value.input stackoutput stack top -> bottomremoved(empty)10(empty)2030

Step 3 - Dequeue pops from output

The output stack pops 10 first while 20 becomes the next front.

After one dequeue: removed is 10; output top is now 20.input stackoutput stack top -> bottomremoved(empty)201030

Basic Implementation

basic.kt 
import java.util.ArrayDeque

fun render(values: List<Int>) = values.joinToString(" -> ")

fun main() {
    val inStack = ArrayDeque<Int>()
    val outStack = ArrayDeque<Int>()
    for (value in listOf(10, 20, 30)) inStack.addLast(value)
    while (!inStack.isEmpty()) outStack.addLast(inStack.removeLast())
    val removed = mutableListOf<Int>()
    while (!outStack.isEmpty()) removed.add(outStack.removeLast())
    println(render(removed))
}

Complexity

  • Time: O(1) amortized per operation
  • Space: O(n)

Implementation notes

  • Kotlin imports java.util.ArrayDeque and creates val inStack and val outStack as ArrayDeque<Int> instances. The bindings are not rebound, but both stack contents mutate.
  • Enqueue writes to the input stack with inStack.addLast(value) for listOf(10, 20, 30), producing [10, 20, 30].
  • Transfer uses while (!inStack.isEmpty()) outStack.addLast(inStack.removeLast()), moving values from the back of inStack to the back of outStack.
  • Dequeue order comes from outStack.removeLast() inside while (!outStack.isEmpty()), so the transferred stack yields 10, 20, then 30.
  • The emptiness checks avoid the exception path for removeLast() on an empty deque; values are non-null Ints throughout.
  • The trace shows in=[] and out=[], then in=[10, 20, 30], then transfer to in=[] and out=[30, 20, 10], then removed [10, 20, 30].
  • render(values: List<Int>) uses joinToString(" -> "), and println(render(removed)) prints 10 -> 20 -> 30.