[LeetCode] Implement Queue using Stacks

A classic interview question. This link has a nice explanation of the idea using two stacks and its amortized time complexity.

I use the same idea in my code, which is as follows.

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if (stack2.empty()) {
            while (!stack1.empty()) {
                int elem = stack1.top();
                stack1.pop();
                stack2.push(elem);
            }
        }
        stack2.pop();
    }

    // Get the front element.
    int peek(void) {
        if (stack2.empty()) {
            while (!stack1.empty()) {
                int elem = stack1.top();
                stack1.pop();
                stack2.push(elem);
            }
        }
        return stack2.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return stack1.empty() && stack2.empty();
    }
private:
    stack<int> stack1;
    stack<int> stack2;
};