[TOC]

设计一个有getMin功能的栈

题目：

• 实现一个特殊的栈，在实现栈的基本功能的基础上，在实现返回栈中最小元素的操作。

要求：

• pop、push、getMin操作的时间复杂度都是O(1)
• 设计的栈可以使用现成的栈结构

题解思路：

题解代码：

import java.util.Stack;

public class Problem_01_GetMinStack {
  public static 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("Your stack is empty.");
      }
      int value = this.stackData.pop();
      if (value == this.getmin()) {
        this.stackMin.pop();
      }
      return value;
    }

    public int getmin() {
      if (this.stackMin.isEmpty()) {
        throw new RuntimeException("Your stack is empty.");
      }
      return this.stackMin.peek();
    }
  }

  public static class MyStack2 {
    private Stack<Integer> stackData;
    private Stack<Integer> stackMin;

    public MyStack2() {
      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);
      } else {
        int newMin = this.stackMin.peek();
        this.stackMin.push(newMin);
      }
      this.stackData.push(newNum);
    }

    public int pop() {
      if (this.stackData.isEmpty()) {
        throw new RuntimeException("Your stack is empty.");
      }
      this.stackMin.pop();
      return this.stackData.pop();
    }

    public int getmin() {
      if (this.stackMin.isEmpty()) {
        throw new RuntimeException("Your stack is empty.");
      }
      return this.stackMin.peek();
    }
  }

  public static void main(String[] args) {
    MyStack1 stack1 = new MyStack1();
    stack1.push(3);
    System.out.println(stack1.getmin());
    stack1.push(4);
    System.out.println(stack1.getmin());
    stack1.push(1);
    System.out.println(stack1.getmin());
    System.out.println(stack1.pop());
    System.out.println(stack1.getmin());

    System.out.println("=============");

    MyStack1 stack2 = new MyStack1();
    stack2.push(3);
    System.out.println(stack2.getmin());
    stack2.push(4);
    System.out.println(stack2.getmin());
    stack2.push(1);
    System.out.println(stack2.getmin());
    System.out.println(stack2.pop());
    System.out.println(stack2.getmin());
  }

}