232_ImplementQueueUsingStacks.cpp

/*
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).
*/

class Queue {
    stack<int> st;
public:
    // Push element x to the back of queue.
    void push(int x){
        if(st.empty()){
            st.push(x);
            return;
        }
        int data;
        data = st.top();
        st.pop();
        push(x);
        st.push(data);
        return;
        
    }

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

    // Get the front element.
    int peek(void) {
        return st.top();
    }

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