解

public class Solution {
// 1 2 3 4 -1
Stack<integer> stackAll = new Stack<>();
// 1 1 1 1 -1
Stack<integer> min = new Stack<>();</integer></integer>

public void push(int node) {
    if(!stackAll.isEmpty() && node > min.peek()){
       node = min.peek();
    }

    stackAll.push(node);
    min.push(node);
}

public void pop() {
    stackAll.pop();
    min.pop();
}

public int top() {
    return stackAll.peek();
}

public int min() {
    return min.peek();
}

}