1、題目

定義棧的資料結構，請在該型別中實現一個能夠得到棧最小元素的min函式，在該棧中，呼叫min、push和pop的時間複雜度都是O(1)。

2、Java程式碼

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

    public int pop(){  //彈棧
        minStack.pop();
        return
 
 stack.pop();
    }

    public void push(int num){  //壓棧
        if (minStack.size() <= 0) {
            minStack.push(num);
            return;
        }
        Integer min = minStack.lastElement();
        if (num < min) {
            minStack.push(num);
        } else {
            minStack.push(min);
        }
        stack.push(num);
    }

    public
 
 int min(){
        if (minStack.size() <= 0) {
            return -1;
        }
        return minStack.lastElement();
    }
}

public class CeshiMinStack {
    public static void main(String[] args) {
        MinStack ms = new MinStack();
        ms.push(20);
        System.out.println(ms.min());
        ms.push(15
 
);
        ms.push(11);
        ms.push(2);
        ms.push(0);
        System.out.println(ms.min());
        ms.pop();
        System.out.println(ms.min());
        ms.pop();
        System.out.println(ms.min());
    }
}