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ImplementingQueueUsingStacks.java

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/*Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue.

pop() -- Removes the element from in front of queue.

peek() -- Get the front element.

empty() -- Return whether the queue is empty.

Notes:

You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.

Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.

You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).*/

class MyQueue {

// Push element x to the back of queue.

private Stack<Integer> stack1;

private Stack<Integer> stack2;

public MyQueue() {

// do initialization if necessary

stack1 = new Stack<Integer>();

stack2 = new Stack<Integer>();

}

public void push(int x) {

stack1.push(x);

}

// Removes the element from in front of queue.

public void pop() {

if (stack2.isEmpty()) {

while (!stack1.isEmpty()) {

stack2.push(stack1.pop());

}

}

stack2.pop();

}

// Get the front element.

public int peek() {

if (stack2.isEmpty()) {

while (!stack1.isEmpty()) {

stack2.push(stack1.pop());

}

}

return stack2.peek();

}

// Return whether the queue is empty.

public boolean empty() {

return (stack2.size() == 0 && stack1.size() == 0);

}

}

// Pochman God

class MyQueue {

Stack<Integer> input = new Stack();

Stack<Integer> output = new Stack();

public void push(int x) {

input.push(x);

}

public void pop() {

peek();

output.pop();

}

public int peek() {

if (output.empty())

while (!input.empty())

output.push(input.pop());

return output.peek();

}

public boolean empty() {

return input.empty() && output.empty();

}

}