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 Go 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.go 
package main

import (
	"fmt"
	"strings"
)

func render(values []int) string {
	parts := make([]string, 0, len(values))
	for _, value := range values {
		parts = append(parts, fmt.Sprint(value))
	}
	return strings.Join(parts, " -> ")
}

func main() {
	inStack := []int{}
	outStack := []int{}
	for _, value := range []int{10, 20, 30} { inStack = append(inStack, value) }
	for len(inStack) > 0 { outStack = append(outStack, inStack[len(inStack)-1]); inStack = inStack[:len(inStack)-1] }
	removed := []int{}
	for len(outStack) > 0 { removed = append(removed, outStack[len(outStack)-1]); outStack = outStack[:len(outStack)-1] }
	fmt.Println(render(removed))
}

Complexity

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

Implementation notes

  • Go represents both stacks as []int slices: inStack := []int{} receives enqueues and outStack := []int{} supplies dequeues.
  • Enqueue uses append(inStack, value) for 10, 20, and 30; append may grow the backing array, while the replay-visible state becomes inStack=[10, 20, 30].
  • The transfer loop pops from the end with inStack[len(inStack)-1], appends that value to outStack, and shrinks the input slice with inStack = inStack[:len(inStack)-1], producing outStack=[30, 20, 10].
  • The dequeue loop uses the same end-pop pattern on outStack, appending values to removed as [10, 20, 30]. The len(...) > 0 guards prevent empty-slice indexing.
  • Reslicing changes slice headers and can keep backing arrays alive while a slice is retained; this small replay drains both stack slices immediately.
  • render converts the removed int values with fmt.Sprint, joins them with strings.Join(parts, " -> "), and fmt.Println prints 10 -> 20 -> 30.