Java实现栈和队列面试题

public class Stack {

public Node head;
 public Node current;

//方法：入栈操作
 public void push(int data) {
  if (head == null) {
   head = new Node(data);
   current = head;
  } else {
   Node node = new Node(data);
   node.pre = current;//current结点将作为当前结点的前驱结点
   current = node; //让current结点永远指向新添加的那个结点
  }
 }

public Node pop() {
  if (current == null) {
   return null;
  }

Node node = current; // current结点是我们要出栈的结点
  current = current.pre; //每出栈一个结点后，current后退一位
  return node;

}

class Node {
  int data;
  Node pre; //我们需要知道当前结点的前一个结点

public Node(int data) {
   this.data = data;
  }
 }

public static void main(String[] args) {

Stack stack = new Stack();
  stack.push(1);
  stack.push(2);
  stack.push(3);

System.out.println(stack.pop().data);
  System.out.println(stack.pop().data);
  System.out.println(stack.pop().data);
 }

}

public class Queue {
 public Node head;
 public Node curent;

//方法：链表中添加结点
 public void add(int data) {
  if (head == null) {
   head = new Node(data);
   curent = head;
  } else {
   curent.next = new Node(data);
   curent = curent.next;
  }
 }

//方法：出队操作
 public int pop() throws Exception {
  if (head == null) {
   throw new Exception("队列为空");
  }

Node node = head; //node结点就是我们要出队的结点
  head = head.next; //出队之后，head指针向下移

return node.data;

}

class Node {
  int data;
  Node next;

public Node(int data) {
   this.data = data;
  }
 }

public static void main(String[] args) throws Exception {
  Queue queue = new Queue();
  //入队操作
  for (int i = 0; i < 5; i++) {
   queue.add(i);
  }

//出队操作
  System.out.println(queue.pop());
  System.out.println(queue.pop());
  System.out.println(queue.pop());

}
}

import java.util.Stack;

/**
* Created by smyhvae on 2015/9/9.
*/
public class Queue {

private Stack<Integer> stack1 = new Stack<>();//执行入队操作的栈
 private Stack<Integer> stack2 = new Stack<>();//执行出队操作的栈

//方法：给队列增加一个入队的操作
 public void push(int data) {
  stack1.push(data);

}

//方法：给队列正价一个出队的操作
 public int pop() throws Exception {

if (stack2.empty()) {//stack1中的数据放到stack2之前，先要保证stack2里面是空的（要么一开始就是空的，要么是stack2中的数据出完了），不然出队的顺序会乱的，这一点很容易忘

while (!stack1.empty()) {
    stack2.push(stack1.pop());//把stack1中的数据出栈，放到stack2中【核心代码】
   }

}

if (stack2.empty()) { //stack2为空时，有两种可能：1、一开始，两个栈的数据都是空的；2、stack2中的数据出完了
   throw new Exception("队列为空");
  }

return stack2.pop();
 }

public static void main(String[] args) throws Exception {
  Queue queue = new Queue();
  queue.push(1);
  queue.push(2);
  queue.push(3);

System.out.println(queue.pop());

queue.push(4);

System.out.println(queue.pop());
  System.out.println(queue.pop());
  System.out.println(queue.pop());

}

}

import java.util.ArrayDeque;
import java.util.Queue;

/**
* Created by smyhvae on 2015/9/9.
*/
public class Stack {

Queue<Integer> queue1 = new ArrayDeque<Integer>();
 Queue<Integer> queue2 = new ArrayDeque<Integer>();

//方法：入栈操作
 public void push(int data) {
  queue1.add(data);
 }

//方法：出栈操作
 public int pop() throws Exception {
  int data;
  if (queue1.size() == 0) {
   throw new Exception("栈为空");
  }

while (queue1.size() != 0) {
   if (queue1.size() == 1) {
    data = queue1.poll();
    while (queue2.size() != 0) { //把queue2中的全部数据放到队列一中
     queue1.add(queue2.poll());
     return data;
    }
   }
   queue2.add(queue1.poll());
  }
  throw new Exception("栈为空");//不知道这一行的代码是什么意思
 }

public static void main(String[] args) throws Exception {
  Stack stack = new Stack();

stack.push(1);
  stack.push(2);
  stack.push(3);

System.out.println(stack.pop());
  System.out.println(stack.pop());
  stack.push(4);
 }
}

import java.util.Stack;

/**
* Created by smyhvae on 2015/9/9.
*/
public class MinStack {

private Stack<Integer> stack = new Stack<Integer>();
 private Stack<Integer> minStack = new Stack<Integer>(); //辅助栈：栈顶永远保存stack中当前的最小的元素

public void push(int data) {
  stack.push(data); //直接往栈中添加数据

//在辅助栈中需要做判断
  if (minStack.size() == 0 || data < minStack.peek()) {
   minStack.push(data);
  } else {
   minStack.add(minStack.peek()); //【核心代码】peek方法返回的是栈顶的元素
  }
 }

public int pop() throws Exception {
  if (stack.size() == 0) {
   throw new Exception("栈中为空");
  }

int data = stack.pop();
  minStack.pop(); //核心代码
  return data;
 }

public int min() throws Exception {
  if (minStack.size() == 0) {
   throw new Exception("栈中空了");
  }
  return minStack.peek();
 }

public static void main(String[] args) throws Exception {
  MinStack stack = new MinStack();
  stack.push(4);
  stack.push(3);
  stack.push(5);

System.out.println(stack.min());
 }
}