Implement Queue using Stacks

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

    // Removes the element from in front of queue.
    void pop(void) {
        if (tmp.empty()) {
            while (!st.empty()) {
                tmp.push(st.top());
                st.pop();
            }
        }
        tmp.pop();
    }

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

    // Return whether the queue is empty.
    bool empty(void) {
        if (st.empty() && tmp.empty())
            return true;
        return false;
    }
};