剑指offer30-设计有min函数的栈

注意在加入的时候，比较当前最小值和参数的大小。

class MinStack {
  Stack<Integer> stack;
        Stack<Integer> min;
    /** initialize your data structure here. */
    public MinStack() {
      stack = new Stack();
      min = new Stack();
    }
    public void push(int x) {
        if(stack.isEmpty()){
            stack.add(x);
            min.add(x);
        }
        else{
            min.add(Math.min(x, min.peek()));
            stack.add(x);
        }
    }
    public void pop() {
        if(stack.isEmpty()) return ;
        stack.pop();
        min.pop();
    }
    public int top() {
        return stack.peek();
    }
    public int min() {
        return min.peek();
    }
}