ImplementQueueUsingStacks232.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
 * 
 * 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).
 */


public class ImplementQueueUsingStacks232 {

    class MyQueue {
        private Stack<Integer> st;
        
        /** Initialize your data structure here. */
        public MyQueue() {
            this.st = new Stack<Integer>();
        }
        
        /** Push element x to the back of queue. */
        public void push(int x) {
            Stack<Integer> temp = new Stack<>();
            int preSize = this.st.size();
            for (int i=0; i<preSize; i++) {
                temp.push(this.st.pop());
            }
            this.st.push(x);
            for (int i=0; i<preSize; i++) {
                this.st.push(temp.pop());
            }
        }
        
        /** Removes the element from in front of queue and returns that element. */
        public int pop() {
            return this.st.pop();
        }
        
        /** Get the front element. */
        public int peek() {
            return this.st.peek();
        }
        
        /** Returns whether the queue is empty. */
        public boolean empty() {
            return this.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();
*/