## Implement Queue using Stacks

07/16/2016 Stack Design

## Question

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).

## Solution

Result: Accepted Time: 0 ms

Here should be some explanations.

#include<stack>
using std::stack;

class Queue {
void reset()
{
if(stack_out.empty())
{
while(!stack_in.empty())
{
stack_out.push(stack_in.top());
stack_in.pop();
}
}
}
stack<int> stack_in;
stack<int> stack_out;
public:
// Push element x to the back of queue.

void push(int x) {
stack_in.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
this->reset();
stack_out.pop();
}

// Get the front element.
int peek(void) {
this->reset();
return stack_out.top();
}

// Return whether the queue is empty.
bool empty(void) {
this->reset();
return stack_out.empty();
}
};


Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$