import java.util.*;
import java.util.Stack;

public class Solution {

    Stack<Integer> s1 = new Stack<Integer>();
    //存最小的元素
    Stack<Integer> s2 = new Stack<Integer>();
    
    public void push(int node) {
        s1.push(node);
        if(s2.isEmpty() || s2.peek() > node){
            s2.push(node);
        }else{
            s2.push(s2.peek());
        }
    }
    
    public void pop() {
        s1.pop();
        s2.pop();
    }
    
    public int top() {
        return s1.peek();
    }
    
    public int min() {
        return s2.peek();
    }
}

用了两个Stack,s2存最小的元素，s1、s2同时push()、pop()