题解 | 设计getMin功能的栈

import java.util.*;

class MyStack1 {

    private Stack<Integer> stackData;
    private Stack<Integer> stackMin;

    public MyStack1() {
        this.stackData = new Stack<Integer>();
        this.stackMin = new Stack<Integer>();
    }

    public void push(int newNum) {
        if (this.stackMin.isEmpty()) {
            this.stackMin.push(newNum);
        } else if (newNum <= this.getmin()) {
            this.stackMin.push(newNum);
        }
        this.stackData.push(newNum);
    }

      public int pop() {
        if (this.stackData.isEmpty()) {
            throw new RuntimeException("Stack is empty");
        }
		//获取值不能用pop，pop是弹出，要用peek
		int value = this.stackData.peek();
        if (value == this.getmin()) {
		  //如果弹出栈的元素是最小值，那min栈也要弹出第一个元素，修改记录的最小值
		    this.stackMin.pop();
        }
        this.stackData.pop();
        return value;
    }

    public int getmin() {
	  //如果栈为空，抛出非首检异常
        if (this.stackMin.isEmpty()) {
            throw new RuntimeException("stackmin is empty");
        }
	  //不为空，就返回最小值，stackmin的是存最小值的栈
        return this.stackMin.peek();
    }
}
public class Main{
        public static void main(String[] args){
            Scanner scan = new Scanner(System.in);
            MyStack1 stack1 = new MyStack1();
            int t = scan.nextInt();
            for(int i=0;i<t;i++){
                String op=scan.next();
                if(op.equals("push")){
                    int x= scan.nextInt();
                    stack1.push(x);
                }else if(op.equals("getMin")){
                    System.out.println(stack1.getmin());
                }else if(op.equals("pop")){
                    stack1.pop();
                }
            }
        }
    }