双栈

stack栈保存数据，如果新入栈的数据是当前最小的，则同时入minStack栈
将要出栈的数如果等于minStack栈顶的数，则两个栈同时出栈

public class Solution {
    private Stack<Integer> stack = new Stack<>();
    private Stack<Integer> minStack = new Stack<>();

    public void push(int node) {
        stack.push(node);
        if (minStack.empty() || node<=minStack.peek()){
            minStack.push(node);
        }
    }

    public void pop() {
        if (!stack.empty()){
            int pop = stack.pop();
            if (pop==minStack.peek()){
                minStack.pop();
            }
        }
    }

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

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