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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: $O(1)$