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Implementing Queue Using Stack

Question: How would you use stacks to implement a queue?
Answer: So how could we go about doing this? What if we used two stacks and used one as the incoming stack and the other as the outgoing stack? A queue is a FIFO structure, so when we enqueue something, we need to make sure that it does not get popped off before something that was already there. Similarly, when we dequeue something, we have to make sure that elements that were inserted earlier get ejected first.

What’s better than some code!
Stack in;
Stack out;
void enqueue( int value ) {
    while( !out.isEmpty() ) {
        in.push( out.pop() );
    in.push( value );

int dequeue() {
    while( !in.isEmpty() ) {
        out.push( in.pop() );
    return out.pop();

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Aptitude Test Practice Questions - With Answers

Question 1. Which of the following is least like the others?
A.    cube
B.    sphere
C.    pyramid
D.    circle

D (because the circle is the only two-dimensional figure)

Question 2. Consider a language which uses the following set of characters:
Small set: { a b c }
Large set: { A B C }
Punctuation set: { x y }
This language must follow the following rules:
  1.    A punctuation character must end all series.
  2.    A series can have up to but no more than 4          characters,including punctuation characters.

Does the following series follow all the rules of the language defined above?
  A.    Yes
  B.    No

A (the series has only four characters and ends in a punctuation character)

Question 3. Consider the following flow chart for a customer:

The person in No.1 is:
    A.   Married, with children
    B.   Married, with at least one son
    C.   Unmarried, with at least one daughter
    D.   Unmarried, with at least one son
    E.   Unmarried, with no children