Implement Queue by Two Stacks

39% Accepted

As the title described, you should only use two stacks to implement a queue's actions.

The queue should support push(element), pop() and top() where pop is pop the first(a.k.a front) element in the queue.

Both pop and top methods should return the value of first element.

Have you met this question in a real interview? Yes
Example
push(1)
pop()     // return 1
push(2)
push(3)
top()     // return 2
pop()     // return 2
Challenge
implement it by two stacks, do not use any other data structure and push, pop and top should be O(1) by AVERAGE.

Tags Expand

• LintCode Copyright
• Stack
• Queue

public class Queue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;
    public Queue() {
       // do initialization if necessary
       stack1 = new Stack<Integer>();
       stack2 = new Stack<Integer>();
    }

    public void push(int element) {
        // write your code here
        stack1.push(element);
    }

    public int pop() {
        // write your code here
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }

    public int top() {
        // write your code here
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.peek();
    }
}