public class TestStack {
    //資料陣列
    private static String[] data;

　　//一個指標
    private static int idx = 0;

    public TestStack(int size){
        if(size <= 0){
            throw new IllegalStateException("error size");
        }
        data = new String[size];
    }

    private void resize(){
        if(idx < data.length){
            return;
        }
        String[] old = data;
        data = new String[data.length * 2];
        System.arraycopy(old, 0, data, 0, old.length);
    }

    public void push(String str){
        resize();
        data[idx] = str;
        idx++;
    }

    public String pop(){
        if(idx == 0){
            return null;
        }
        return data[--idx];
    }

    public static void main(String[] args) {
        TestStack stack = new TestStack(1);
        for(int i =0;i <= 19; i++){
            stack.push("a" + i);
        }

        for (int j =0 ; j <= 19;j++){
            String str = stack.pop();
            System.out.println(str);
        }
    }
}