232.implement-queue-using-stacks.java

// 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.
// Example:

// MyQueue queue = new MyQueue();

// queue.push(1);
// queue.push(2);  
// queue.peek();  // returns 1
// queue.pop();   // returns 1
// queue.empty(); // returns false

class MyQueue {
    Stack<Integer> st;
    /** Initialize your data structure here. */
    public MyQueue() {
        st = new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        if (st.isEmpty()) {
            st.push(x);
            return;
        }
        int temp = st.pop();
        push(x);
        st.push(temp);
    }
    
    // private void (i)
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return st.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        return st.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return st.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */