实现一个栈，在基本功能的基础上，可以返回栈中最小值

import java.util.Stack;

/**
 * 实现一个栈，在基本功能的基础上，可以返回栈中最小值
 */
public class GetMinStack {

    public static class MyStack {

        public Stack<Integer> stackData;

        public Stack<Integer> stackMin;

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

        public void push(Integer value) {
            if (stackMin.isEmpty()) {
                stackMin.push(value);
            } else if (value <= getMin()) {
                stackMin.push(value);
            } else {
                stackMin.push(getMin());
            }
            stackData.push(value);
        }

        public Integer pop() {
            if (stackData.isEmpty()) {
                System.out.println("the stack is empty");
                return null;
            }
            stackMin.pop();
            return stackData.pop();
        }

        public Integer getMin() {
            if (stackMin.isEmpty()) {
                System.out.println("the stack is empty");
                return null;
            }
            return stackMin.peek();
        }

    }

    public static class MyStack2 {

        public Stack<Integer> stackData;

        public Stack<Integer> stackMin;

        public MyStack2() {
            this.stackData = new Stack<>();
            this.stackMin = new Stack<>();
        }

        public void push(Integer value) {
            if (stackMin.isEmpty()) {
                stackMin.push(value);
            } else if (value <= getMin()) {
                stackMin.push(value);
            }
            stackData.push(value);
        }

        public Integer pop() {
            if (stackData.isEmpty()) {
                System.out.println("the stack is empty");
                return null;
            }
            if (stackData.peek() == getMin()) {
                stackMin.pop();
            }
            return stackData.pop();
        }

        public Integer getMin() {
            if (stackMin.isEmpty()) {
                System.out.println("the stack is empty");
                return null;
            }
            return stackMin.peek();
        }

    }

}

/* 如有错误，欢迎批评指正 */