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 input;
stack 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();
}
};