How to Fix Queue Using Stacks Errors

In this tutorial, you'll learn about How to Fix Queue Using Stacks Errors. We cover key concepts, practical examples, and best practices.

Fix queue using stacks errors when amortized analysis fails or elements processed out of order.

Quick Fix

Wrong

class Q:
    def __init__(self): self.s1=[]; self.s2=[]
    def enq(self,x):
        while self.s1: self.s2.append(self.s1.pop())
        self.s1.append(x)
        while self.s2: self.s1.append(self.s2.pop())
    def deq(self): return self.s1.pop()

enqueue is O(n) with double transfer.

Right

class Q:
    def __init__(self): self.i=[]; self.o=[]
    def enq(self,x): self.i.append(x)
    def deq(self):
        if not self.o:
            while self.i: self.o.append(self.i.pop())
        return self.o.pop()
    def peek(self):
        if not self.o:
            while self.i: self.o.append(self.i.pop())
        return self.o[-1]
    def empty(self): return not self.i and not self.o

enq(1),enq(2),deq()->1,enq(3),deq()->2. Amortized O(1).

Prevention

Input for enqueue, output for dequeue. Transfer only when output empty.

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