剑指 Offer 09. 用两个栈实现队列

class CQueue {

    private final Stack<Integer> inStack;
    private final Stack<Integer> outStack;

    public CQueue() {
        this.inStack = new Stack<>();
        this.outStack = new Stack<>();
    }

    public void appendTail(int value) {
        inStack.push(value);
    }

    public int deleteHead() {
        if (!outStack.isEmpty()) {
            return outStack.pop();
        } else {
            while (!inStack.isEmpty()) {
                outStack.push(inStack.pop());
            }

            return outStack.isEmpty() ? -1 : outStack.pop();
        }
    }

}

Reference