Javase复习day21算法、arrays、Lamdba表达式

常见算法

查找算法

基本查找

package search;

public class BasicSearchDemo1 {
    public static void main(String[] args) {
        //基本算法（顺序查找）
        int[] arr = {131,23,57,37,95,48,57,43};
        System.out.println(basicSearch(arr, 43));
    }
    public static boolean basicSearch(int[] arr,int number){
        for (int i = 0; i < arr.length; i++) {
            if (arr[i]==number){
                return true;
            }
        }
        return false;
    }
}

课堂练习1

课堂练习2

package search;

import java.util.ArrayList;

public class BasicSearchDemo2 {
    public static void main(String[] args) {
        //基本算法（顺序查找）
        int[] arr = {131,23,57,37,95,48,57,43,23};
        System.out.println(basicSearch1(arr, 43));
        System.out.println(basicSearch2(arr, 23));
    }
    public static int basicSearch1(int[] arr,int number){
        for (int i = 0; i < arr.length; i++) {
            if (arr[i]==number){
                return i;
            }
        }
        return -1;
    }
    public static ArrayList<Integer> basicSearch2(int[] arr, int number){
        ArrayList<Integer> list = new ArrayList<>();
        for (int i = 0; i < arr.length; i++) {
            if (arr[i]==number){
                list.add(i);
            }
        }
        return list;
    }
}

二分查找（折半查找）

package search;

public class BinarySearchDemo1 {
    public static void main(String[] args) {
        //二分查找
        int[] arr = {7,23,79,81,103,127,131,147};
        System.out.println(basicSearch(arr, 127));
    }
    public static int basicSearch(int[] arr,int number){
        int min = 0;
        int max = arr.length;
        while (max>=min){
            int mid = (min+max)/2;
            if (number>arr[mid]){
                min = mid+1;
            } else if (number<arr[mid]) {
                max = mid-1;
            }else {
                return mid;
            }
        }
        return -1;
    }
}

二分查找小结

二分查找的改进

插值查找

斐波那契查找

二分查找的小结

分块查找

package search;

public class BlockSearchDemo {
    public static void main(String[] args) {
        //初步将其分块
        int[] arr = {16,5,9,12, 21,18
                ,32,23,37,26,45,34
                ,50,48,61,52,73,66};
        Block b1 = new Block(21,0,5);
        Block b2 = new Block(45,6,11);
        Block b3 = new Block(73,12,17);

        //定义数组来管理
        Block[] blocks = {b1,b2,b3};
        //记录要查找的数据

        int number = 73;

        //调用方法传递索引表、数组、查询的元素
        int index = getIndex(blocks,arr,number);
        System.out.println(index);
    }

    private static int getIndex(Block[] blocks, int[] arr, int number) {
        int indexBock = findIndexBock(blocks, number);
        if (indexBock==-1){
            //表示没有返回-1
            return -1;
        }
        //将对应块的起始索引和结束索引取出来
        int startIndex = blocks[indexBock].getStartIndex();
        int endIndex = blocks[indexBock].getEndIndex();
        //遍历到则存在
        for (int i = startIndex; i <= endIndex; i++) {
            if (arr[i]==number){
                return i;
            }
        }
        return -1;
    }
    //定义一个方法用来确认number的块
    public static int findIndexBock(Block[] blocks,int number){
        //二分查找进行操作
        int min = 0;
        int max = blocks.length;
        while (max>=min){
            int mid = (min+max)/2;
            if (number>blocks[mid].getMax()){
                min = mid+1;
            } else if (number<=blocks[mid].getMax()) {
                return mid;
            }
        }
        return -1;
        //顺序查找
//        for (int i = 0; i < blocks.length; i++) {
//            if (blocks[i].getMax()>=number){
//                return i;
//            }
//        }
//        return -1;
    }

}
//定义一个类记录分块情况
class Block{
    private int max;
    private int startIndex;
    private int endIndex;

    public Block() {
    }

    public Block(int max, int startIndex, int endIndex) {
        this.max = max;
        this.startIndex = startIndex;
        this.endIndex = endIndex;
    }

    /**
     * 获取
     * @return max
     */
    public int getMax() {
        return max;
    }

    /**
     * 设置
     * @param max
     */
    public void setMax(int max) {
        this.max = max;
    }

    /**
     * 获取
     * @return startIndex
     */
    public int getStartIndex() {
        return startIndex;
    }

    /**
     * 设置
     * @param startIndex
     */
    public void setStartIndex(int startIndex) {
        this.startIndex = startIndex;
    }

    /**
     * 获取
     * @return endIndex
     */
    public int getEndIndex() {
        return endIndex;
    }

    /**
     * 设置
     * @param endIndex
     */
    public void setEndIndex(int endIndex) {
        this.endIndex = endIndex;
    }

    public String toString() {
        return "Block{max = " + max + ", startIndex = " + startIndex + ", endIndex = " + endIndex + "}";
    }
}

扩展的分块查找（无规律的数据）

package search;

public class BlockSearchDemo2 {
    public static void main(String[] args) {
        //初步将其分块
        int[] arr = {27,22,30,40,36,
                13,19,16,20,
                7,10,
                43,50,48};
        Block2 b1 = new Block2(22,40,0,4);
        Block2 b2 = new Block2(13,20,5,8);
        Block2 b3 = new Block2(7,10,9,10);
        Block2 b4 = new Block2(43,50,11,13);

        //定义数组来管理
        Block2[] blocks = {b1,b2,b3,b4};
        //记录要查找的数据

        int number = 40;

        //调用方法传递索引表、数组、查询的元素
        int index = getIndex(blocks,arr,number);
        System.out.println(index);
    }

    private static int getIndex(Block2[] blocks, int[] arr, int number) {
        int indexBock = findIndexBock(blocks, number);
        if (indexBock==-1){
            //表示没有返回-1
            return -1;
        }
        //将对应块的起始索引和结束索引取出来
        int startIndex = blocks[indexBock].getStartIndex();
        int endIndex = blocks[indexBock].getEndIndex();
        //遍历到则存在
        for (int i = startIndex; i <= endIndex; i++) {
            if (arr[i]==number){
                return i;
            }
        }
        return -1;
    }
    //定义一个方法用来确认number的块
    public static int findIndexBock(Block2[] blocks,int number){

//        由于分布没有顺序不能使用二分查找所以使用顺序查找
        for (int i = 0; i < blocks.length; i++) {
            if (blocks[i].getMax()>=number&&blocks[i].getMin()<=number){
                return i;
            }
        }
        return -1;
    }

}
//定义一个类记录分块情况
class Block2{
    private int max;
    private int min;
    private int startIndex;
    private int endIndex;

    public Block2() {
    }

    public Block2( int min,int max, int startIndex, int endIndex) {
        this.max = max;
        this.min = min;
        this.startIndex = startIndex;
        this.endIndex = endIndex;
    }

    /**
     * 获取
     * @return max
     */
    public int getMax() {
        return max;
    }

    /**
     * 设置
     * @param max
     */
    public void setMax(int max) {
        this.max = max;
    }

    /**
     * 获取
     * @return min
     */
    public int getMin() {
        return min;
    }

    /**
     * 设置
     * @param min
     */
    public void setMin(int min) {
        this.min = min;
    }

    /**
     * 获取
     * @return startIndex
     */
    public int getStartIndex() {
        return startIndex;
    }

    /**
     * 设置
     * @param startIndex
     */
    public void setStartIndex(int startIndex) {
        this.startIndex = startIndex;
    }

    /**
     * 获取
     * @return endIndex
     */
    public int getEndIndex() {
        return endIndex;
    }

    /**
     * 设置
     * @param endIndex
     */
    public void setEndIndex(int endIndex) {
        this.endIndex = endIndex;
    }

    public String toString() {
        return "Block{max = " + max + ", min = " + min + ", startIndex = " + startIndex + ", endIndex = " + endIndex + "}";
    }
}

扩展的分块查找（查找过程中还需要添加数据）

排序算法

冒泡排序

package sort;

public class BubbleDemo1 {
    public static void main(String[] args) {
        int[] arr = {2,4,5,3,1};
        int[] arr2 = bubble(arr);
        for (int i = 0; i < arr2.length; i++) {
            System.out.print(arr2[i]+" ");
        }
    }

    private static int[] bubble(int[] arr) {
        //表示执行多少轮
        for (int j = 0; j < arr.length-1; j++) {
            /**
             * 每一轮如何找到该轮的最大值
             * -1 表示 防止索引越界
             *  -j 提高效率，已经排序的位置不需要再比较
             */
            for (int i = 0; i < arr.length -1-j; i++) {
                if (arr[i] > arr[i+1]){
                    int temp = arr[i];
                    arr[i] = arr[i+1];
                    arr[i+1] = temp;
                }
            }
        }
        return arr;
    }
}

选择排序

package sort;

public class SelectionDemo {
    public static void main(String[] args) {
        int[] arr = {2,10,5,3,1};
        int[] arr2 = selection(arr);
        for (int i = 0; i < arr2.length; i++) {
            System.out.print(arr2[i]+" ");
        }
    }

    private static int[] selection(int[] arr) {
        //使用0索引与后面比较
        for (int i = 0; i < arr.length-1; i++) {
            //int j = i+1减少不必要的比较提高效率
            for (int j = i+1; j < arr.length; j++) {
                if (arr[i]>arr[j]){
                    int temp= arr[i];
                    arr[i] = arr[j];
                    arr[j] = temp;
                }
            }
        }
        return arr;
    }

}

插入排序

交换位置时从后往前进行遍历，最终确定自己的位置。

package sort;

public class InsertDemo {
    //插入排序
    public static void main(String[] args) {
        int[] arr = {1,5,10,3,20,7,40,54,4,6,23,47};
        int[] arr2 = inserta(arr);
        for (int i = 0; i < arr2.length; i++) {
            System.out.print(arr2[i]+" ");
        }
    }

    private static int[] inserta(int[] arr) {
        //快速排序
        //先确定无序的位置,无序的起始索引
        int start = -1;
        for (int i = 0; i < arr.length; i++) {
            if (arr[i]>arr[i+1]){
                start = i+1;
                break;
            }
        }

        for (int i = start; i < arr.length; i++) {
            //j 进行操作防止i 改变后导致程序无法运行
            int j = i;
            //当前一个数比当前的数大时两数交换位置，并将指针即j前移一位
            while (j>0&&arr[j]<arr[j-1]){
                int temp = arr[j];
                arr[j] = arr[j-1];
                arr[j-1]=temp;
                j--;
            }
        }
        return arr;
    }
}

快速排序

递归

注意

递归一定要有出口，否则就会内存溢出。

作用

练习

package sort;



public class RecursionDemo2 {
    //求1-100的和
    public static void main(String[] args) {
        int n =100;
        int sun = sum1(100);
        System.out.println(sun);
    }

    private static int sum1(int i) {
        if (i==1){
           return 1;
        }
        return i+sum1(i-1);
    }
}

练习二

package sort;



public class RecursionDemo3 {
    //求5的阶乘
    public static void main(String[] args) {
        int factorial = factorial(5);
        System.out.println(factorial);
    }

    private static int factorial(int i) {
        if (i==1){
           return 1;
        }
        return i*factorial(i-1);
    }
}

快速排序

package sort;

public class QuickDemo {
    public static void main(String[] args) {
        //快速排序
        int[] arr = {7,4,2,8,5,9,12,35,45,98,34,76};

        quickSort(arr, 0, arr.length-1);
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i]+" ");
        }

    }

    private static void quickSort(int[] arr, int i, int j) {

        //快排第一轮
        int start = i;
        int end = j;
        //设置递归的条件
        if(start>end){
            return;
        }
        int Benchmark = arr[i];
        while (start != end){
            //先寻找比基准数小的，用end记录下标
            while (true){
                if (end<=start||arr[end]<Benchmark){
                    break;
                }
                end--;
            }
            //在寻找比基准数大的，用start记录下标
            while (true){
                if (end<=start||arr[start]>Benchmark){
                    break;
                }
                start++;
            }
            //换位值
            int temp = arr[start];
            arr[start] = arr[end];
            arr[end] = temp;

        }
        int temp = arr[i];
        arr[i] = arr[end];
        arr[end] = temp;
        //开始进行递归调用
        quickSort(arr,i,start-1);
        quickSort(arr,start+1,j);

    }
}

排序总结

Arrays

arrays即操作数组的工具类。

方法

sort中的指定规则排序

package ArraysDemo;


import java.util.Arrays;
import java.util.Comparator;

public class ArraysDemo1 {
    public static void main(String[] args) {
        int[] arr = {1,2,3,4,5,6,7,8,9};
        System.out.println(Arrays.toString(arr));

        System.out.println(Arrays.binarySearch(arr, 4));
        System.out.println(Arrays.binarySearch(arr, 10));
        System.out.println(Arrays.binarySearch(arr, 0));

        System.out.println("=======================");
        int[] ints = Arrays.copyOf(arr, 4);
        System.out.println(Arrays.toString(ints));

        int[] ints1 = Arrays.copyOf(arr, 9);
        System.out.println(Arrays.toString(ints1));

        int[] ints2 = Arrays.copyOfRange(arr, 0, 9);
        System.out.println(Arrays.toString(ints2));
        System.out.println("=========================");

//        Arrays.fill(arr,1);
//        System.out.println(Arrays.toString(arr));
//        Arrays.sort(arr,new Comparator<>());
        //o2-o1为降序排列，o1-o2为升序排列
        Integer[] arr2 = {1,2,3,4,5,6,7,8,9};
        Arrays.sort(arr2, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o2-o1;
            }
        });
        System.out.println(Arrays.toString(arr2));

    }
}

Lambda表达式

试水

package LambdaDemo;

import java.util.Arrays;
import java.util.Comparator;

public class Demo1 {
    public static void main(String[] args) {
        Integer[] arr = {1,2,3,7,4,9,6};

//        Arrays.sort(arr, new Comparator<Integer>() {
//            @Override
//            public int compare(Integer o1, Integer o2) {
//                return o1=o2;
//            }
//        });
        Arrays.sort(arr, (Integer o1, Integer o2) -> {
                return o1=o2;
            }
        );
        System.out.println(Arrays.toString(arr));
    }
}

函数式编程

package LambdaDemo;

public class Demo2 {
    public static void main(String[] args) {
        //正常情况
        method(new Swin() {
            @Override
            public void swimming() {
                System.out.println("匿名内部类游泳~");
            }
        });

        //lambda表达式
        method(()->{
            System.out.println("lambda游泳~");
        });
    }
    public static void method(Swin swin){
        swin.swimming();
    }
}
interface Swin{
    public void swimming();
}

Lambda表达式的省略写法

核心思想：可推导，可省略。

package LambdaDemo;

import java.util.Arrays;
import java.util.Comparator;

public class Demo3 {
    public static void main(String[] args) {
        //正常情况
        method(new Swin1() {
            @Override
            public void swimming() {
                System.out.println("匿名内部类游泳~");
            }
        });

        //lambda表达式
        method(()->{
            System.out.println("lambda游泳~");
        });

        //进行省略写法
        method(()-> System.out.println("lambda游泳省略写法~"));

        //多个形参的案例
        Integer[] arr2 = {1,2,3,4,5,6,7,8,9};
        //形参类型相同可以推导出来，省略
        //将大括号，return、分号一起省略。
        Arrays.sort(arr2, (o1,o2) -> o2-o1);
        System.out.println(Arrays.toString(arr2));

    }
    public static void method(Swin1 swin){
        swin.swimming();
    }
}
interface Swin1{
    public void swimming();
}

练习一：Lambda表达式简化Comparator接口的匿名形式

package LambdaDemo;

import java.util.Arrays;
import java.util.Comparator;

public class Demo4 {
    public static void main(String[] args) {
        String[] arr ={"aa","a","aaaa","aaa"};
        Arrays.sort(arr,( o1,o2) ->o1.length()-o2.length());
        System.out.println(Arrays.toString(arr));
    }
}

综合练习

练习一：按要求排序

package Test.Test1;

import java.util.Arrays;
import java.util.Comparator;

public class Test1 {
    public static void main(String[] args) {
        //创建几个对象
        Person p1 = new Person("xiaoli",20,175);
        Person p2 = new Person("aiaoli",20,175);
        Person p3 = new Person("biaoli",19,172);
        Person p4 = new Person("ciaoli",22,168);
        Person p5 = new Person("diaoli",20,168);
        Person p6 = new Person("eiaoli",20,165);
        Person p7 = new Person("fiaoli",20,175);
        //定义数组进行存储
        Person[] arr={p1,p2,p3,p4,p5,p6,p7};
        //进行排序
        //匿名内部类写法
//        Arrays.sort(arr, new Comparator<Person>() {
//            @Override
//            public int compare(Person o1, Person o2) {
//                //o1为要加入的
//                if (o1.getAge()>o2.getAge()){
//                    return 1;
//                } else if (o1.getAge()<o2.getAge()) {
//                    return -1;
//                }else {
//                    //年龄一样，按身高排序
//                    if (o1.getHight()>o2.getHight()){
//                        return 1;
//                    } else if (o1.getHight()<o2.getHight()) {
//                        return -1;
//                    }else {
//                        //按姓名的字母排序
//                        //获取姓名的首字母
                        char o1n = o1.getName().charAt(0);
                        char o2n = o2.getName().charAt(0);
//                        //可以使用字符串的比较方法来比较
//                      return o1.getName().compareTo(o2.getName());
                        if (o1n>o2n){
                            return 1;
                        } else if (o1n<o2n) {
                            return -1;
                        }
//                    }
//
//                }
//            }
//        });
        //Lambda表达式写法
        Arrays.sort(arr, (o1,o2) ->{
                //o1为要加入的
                if (o1.getAge()>o2.getAge()){
                    return 1;
                } else if (o1.getAge()<o2.getAge()) {
                    return -1;
                }else {
                    //年龄一样，按身高排序
                    if (o1.getHight()>o2.getHight()){
                        return 1;
                    } else if (o1.getHight()<o2.getHight()) {
                        return -1;
                    }else {
                        //按姓名的字母排序
                        //可以使用字符串的比较方法来比较
                        return o1.getName().compareTo(o2.getName());
                    }
                }
            }
        );
        //将数组转为字符串进行输出
        System.out.println(Arrays.toString(arr));
    }
}

实现方法二（教学的教简单）：

package Test.Test1;

import java.util.Arrays;
import java.util.Comparator;

public class Test2 {
    public static void main(String[] args) {
        //创建几个对象
        Person p1 = new Person("xiaoli",20,175);
        Person p2 = new Person("aiaoli",20,175);
        Person p3 = new Person("biaoli",19,172);
        Person p4 = new Person("ciaoli",22,168);
        Person p5 = new Person("diaoli",20,168);
        Person p6 = new Person("eiaoli",20,165);
        Person p7 = new Person("fiaoli",20,175);
        //定义数组进行存储
        Person[] arr={p1,p2,p3,p4,p5,p6,p7};
        //进行排序
        //匿名内部类写法
//        Arrays.sort(arr, new Comparator<Person>() {
//            @Override
//            public int compare(Person o1, Person o2) {
//                //比较年龄
//                double temp = o1.getAge()-o2.getAge();
//                //年龄相对，比较身高
//                temp = temp == 0? o1.getHight()-o2.getHight():temp;
//                //身高相等，比较姓名
//                temp = temp == 0? o1.getName().compareTo(o2.getName()):temp;
//                
//                //由于返回值类型为int所以进行判断
//                
//                if (temp<0){
//                    return -1;
//                } else if (temp>0) {
//                    return 1;
//                }else {
//                    return 0;
//                }
//            }
//        });
        //Lambda表达式写法
        Arrays.sort(arr, (o1,o2) -> {
                //比较年龄
                double temp = o1.getAge()-o2.getAge();
                //年龄相对，比较身高
                temp = temp == 0? o1.getHight()-o2.getHight():temp;
                //身高相等，比较姓名
                temp = temp == 0? o1.getName().compareTo(o2.getName()):temp;

                //由于返回值类型为int所以进行判断

                if (temp<0){
                    return -1;
                } else if (temp>0) {
                    return 1;
                }else {
                    return 0;
                }
            }
        );
        //将数组转为字符串进行输出
        System.out.println(Arrays.toString(arr));
    }
}

练习二：不死神兔

从第三个月开始这个月的数量都是前两个月的合。

package Test.Test2;

public class Test {
    public static void main(String[] args) {
        //从第三个数据开始每个数据都是前两个数据之和
        //解法一：使用数组
//        int[] arr = new int[12];
//        arr[0]=1;
//        arr[1]=1;
//        for (int i = 2; i < arr.length; i++) {
//            arr[i] = arr[i-1]+arr[i-2];
//        }
//        System.out.println(arr[11]);
        //方法二：使用递归
        System.out.println(fi(12));
    }
    public static int fi(int n){
        if (n==1||n==2){
            return 1;
        }else {
            return fi(n-1)+fi(n-2);
        }
    }


}

练习三：猴子吃桃子

package Test.Test3;

public class Test3 {
    public static void main(String[] args) {
        System.out.println(fi(1));
    }
    public static int fi(int n){
        //进行校验
        if (n>=11||n<=0){
            return -1;
        }
        if (n==10){
            return 1;
        }else {
            return (fi(n+1)+1)*2;
        }
    }
}