解法一：负负得正

import java.util.Stack;

public class Solution {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();

    public void push(int node) {
        stack1.push(node);
    }

    public int pop() {
        while(!stack1.isEmpty()) stack2.push(stack1.pop());
        int t=stack2.pop();
        while(!stack2.isEmpty()) stack1.push(stack2.pop());
        return t;
    }
}

解法二：优化

import java.util.Stack;

public class Solution {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();

    public void push(int node) {
        stack1.push(node);
    }

    public int pop() {
        if(!stack2.isEmpty())
            return stack2.pop();
        while(!stack1.isEmpty()) stack2.push(stack1.pop());
        return stack2.pop();        
    }
}