思路： 需要一个辅助栈来存放每次push操作时，当前栈内元素的最小值。

Paste_Image.png

Paste_Image.png

public class Solution {
    Stack<Integer> data_stack=new Stack<>();
    Stack<Integer> min_stack=new Stack<>();
    int min=Integer.MAX_VALUE;
    
    public void push(int node) {
        data_stack.push(node);
        min=Math.min(min,node);
        if(node<min){
            min_stack.push(node);
        }else{
            min_stack.push(min);
        }
    }
    
    public void pop() {
        min_stack.pop();
        data_stack.pop();
    }
    
    public int top() {
        return data_stack.peek();
    }
    
    public int min() {
        return min_stack.peek();
    }
}