30、包含min函数的栈

题目：定义栈的数据结构，请在该类型中实现一个能够得到栈中所含最小元素的min函数（时间复杂度应为O（1））。

https://www.nowcoder.com/practice/4c776177d2c04c2494f2555c9fcc1e49

import java.util.Stack;

public class Solution {

    private Stack<Integer> minStack = new Stack();
    private Stack<Integer> normalStack = new Stack();
    
    public void push(int node) {
        normalStack.push(node);
        if(minStack.isEmpty() || node<=minStack.peek()){
            minStack.push(node);
        }else{
            minStack.push(minStack.peek());
        }
    }
    
    public void pop() {
        if(normalStack.isEmpty()) return;
        normalStack.pop();
        minStack.pop();
    }
    
    public int top() {
        return normalStack.peek();
    }
    
    public int min() {
        return minStack.peek();
    }
}