class MinStack {
    //定义两个栈，stack和minStack
    private Stack<Integer> stack = new Stack<>();
    private Stack<Integer> minStack = new Stack<>();
    public MinStack() {
    }
    public void push(int val) {
       stack.push(val);
       if(minStack.isEmpty()) {
           //先将第一个元素放入stack2当中
           minStack.push(val);
       }else {
            //注意这块需要写等于号
           if(val <= minStack.peek()) {
               minStack.push(val);
          }
       }
    }
    public void pop() {
        int x = stack.pop();
        if(x == minStack.peek()) {
            minStack.pop();
        }
    }
    public int top() {
       return stack.peek();
    }
    public int min() {
       return minStack.peek();
    }
}