we could use selection sort which needs two more stack
we could use insertion sort which needs one more stack.

//implement insertion sort 
public class Solution {
    public void sort(Stack<Integer> s) {
        Stack<Integer> s2 = new Stack<Integer>();
        while (!s.isEmpty()) {
            // Insert each element in s in sorted order into s2(nondescending order)
            int tmp = s.pop();
            while (!s2.isEmpty() && s2.peek() > tmp) {
                s.push(s2.pop());
            }
            s2.push(tmp);
        }
        
        //Copy the elements from s2 back into s.
        while (!s2.isEmpty()) {
            s.push(s2.pop());
        }
    }
}