Implement Queue using Stacks

Description:

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:

class Queue {
private:
	stack<int> input;
	stack<int> output;

public:
	// Push element x to the back of queue.
	void push(int x) {
    	input.push(x);
	}

	// Removes the element from in front of queue.
	void pop(void) {
    	peek();
    	output.pop();
    }

	// Get the front element.
    int peek(void) {
	    if (output.empty()) {
    	    while (!input.empty()) {
        	    output.push(input.top());
            	input.pop();
            }
	    }
    	return output.top();
    }

	// Return whether the queue is empty.
    bool empty(void) {
	    return input.empty() && output.empty();
    }
};