算法之设计一个有getMin方法的栈结构

Posted 2020-09-05

要求该栈的getMin方法和push和pop方法的时间复杂度都是O(1).

设计方案一：

package coding.chapter01;

import java.util.Stack;

/**
 * Created by Needle on 2017/3/17.
 */
public class GetMinStack {
    private Stack<int> dataStack;//存储数据的栈
    private Stack<int> minStack;//存储最小值的栈

    public GetMinStack(){
        dataStack = new Stack<int>();
        minStack = new Stack<int>();
    }

    public int getMin(){
        if (minStack.isEmpty())
            throw new RuntimeException("The Stack is empty....");
        else
            return minStack.peek();//获取栈顶元素的值，并不出栈

    }

    public void push(int num){
        //对minStack操作
        if (minStack.isEmpty())
            minStack.push(num);
        else if (num <= getMin())
            minStack.push(num);
        //对dataStack进行操作
        dataStack.push(num);
    }

    public int pop(){
        if (minStack.isEmpty())
            throw new RuntimeException("The Stack is empty....");
        int value = dataStack.pop();//dataStack正常出栈
        if (value == getMin())//minStack仅仅在于相等的情况下才出栈
            minStack.pop();
        return value;
    }
}

 设计方案二：

package coding.chapter01;

import java.util.Stack;

/**
 * Created by Needle on 2017/3/19.
 */
public class GetMinStack2 {
    private Stack<int> dataStack;//保存数据的栈
    private Stack<int> minStack;//保存最小值的栈

    public GetMinStack2(Stack<int> dataStack, Stack<int> minStack) {
        this.dataStack = dataStack;
        this.minStack = minStack;
    }

    //出栈
    public int pop(){
        if (dataStack.isEmpty())
            throw new RuntimeException("Error: Stack is empty!");
        int value = dataStack.pop();
        minStack.pop();
        return value;
    }

    public void push(int num){
        dataStack.push(num);
        if(minStack.isEmpty()){
            minStack.push(num);
        }
        else if(num <= minStack.peek())
            minStack.push(num);
        else
            minStack.push(minStack.peek());
    }

    public int GetMin2(){
        if (minStack.isEmpty())
            throw new RuntimeException("Error: Stack is empty!");
        return minStack.peek();
    }
}