编程算法题总结

一.常见的排序算法

冒泡排序:

public class maoPao {public static void main(String[] args) {int a[]= {7,8,5,6,10};int b[]=maoPao(a);for (int i : b) {System.out.println(i);}}public static int[] maoPao(int[] a) {int temp;for (int i = 0; i < a.length-1; i++) {for (int j = 0; j < a.length-i-1; j++) {if (a[j]>=a[j+1]) {temp=a[j];a[j]=a[j+1];a[j+1]=temp;}System.out.println(a[j]);}}return a;}}

选择排序

//选择排序==>比较相邻的元素。如果第一个比第二个大，就交换他们两个。 public class PaiXu {public static void main(String[] args) {int a[]= {7,8,5,6,10};int b[]=xuanZe(a);for (int i : b) {System.out.println(i);}}public static int[]  xuanZe(int[] a) {int temp;//设置中间值for (int i = 0; i < a.length-1; i++) {for (int j = i+1; j < a.length; j++) {if (a[i]>=a[j]) {temp=a[i];a[i]=a[j];a[j]=temp;}}}return a;}}

3.数组反转

方法一:直接将数组中的最前面的数与最后面的数依次交换

public class Paixu1 {public static void main(String[] args) {char[] a= {'a','1','我','k','2'};int len=a.length;for(int i=0;i<len/2;i++){char tmp = a[i];a[i] = a[len-1-i];a[len-1-i] = tmp;}for (char c : a) {System.out.println(c);}}}

方法二:直接加入list集合中,利用集合自带的reserve方法进行反转 

public class Paixu1 {public static void main(String[] args) {char[] a= {'a','1','我','k','2'};List  list=new ArrayList<>();for (Object o : a) {list.add(o);}Collections.reverse(list);System.out.println(list);}}

4.单例模式

饿汉模式:

 

public class Singleton{private static Singleton singleton=new Singleton();private Singleton() {}private static Singleton getSingleton() {return singleton;}}

懒汉模式

class Singleton1{private static Singleton1 singleton1;private Singleton1() {}private static Singleton1 getSingleton1() {if (singleton1==null) {singleton1=new Singleton1();}return singleton1;}}

五.统计一个字符串中,中文,英文与数字的个数

public class StringTrim {public static void main(String[] args) {String  str="ABCabc我爱你123";int countNum=0;//统计数字的个数int countChinese=0;//统计中文字符个数int countEnglish=0;//统计英文字符的个数//1.首先将str转为字符数组char[] c=str.toCharArray();//2.遍历字节数组进行判断for (char d : c) {if (d>='0'&&d<='9') {countNum++;}else if ((d>='a'&&d<='z')||(d>='A'&&d<='Z')) {countEnglish++;}else {countChinese++;}}System.out.println("中文的字符个数:"+countChinese);System.out.println("数字的字符个数:"+countNum);System.out.println("英文的个数:"+countEnglish);}}

六.//一个整数，大于0，不用循环和本地变量，按照 n，2n，4n，8n 的顺序递增，当值大于5000

//时，把值按照指定顺序输出来。

//例：n=1237

//则输出为：

//1237，

//2474，

//4948，

//9896，

//9896，

//4948，

//2474，

//1237，

public class Test1 {public static void main(String[] args) {System.out.println("请输入一个小于5000的数字");Scanner scanner=new Scanner(System.in);int num=scanner.nextInt();getNum(num);}public static int  getNum(int num) {if (num/2<=5000) {System.out.println(num);getNum(num*2);System.out.println(num);}return num;}public static void  getNum1(int num) {}}

六.向上转型与向下转型

public class Test2 {public static void main(String[] args) {Person person=new Student();//向上转型person.work();//输出学生在学习person.play();//这个方法是获取不到的Student student=(Student) person;//向下转型student.play();//输出学生在玩student.work();//输出学生在学习}}class Person{String type;//人的种类void work() {System.out.println("人类在工作");}}class Student extends Person{String name;//学生姓名void work() {System.out.println("学生在学习");//覆盖父类的方法}void play() {System.out.println("学生在玩");//定义自身的方法}}

七.分解质因数

public class Test3 {public static void main(String[] args) {getNum(1000);}public static void getNum(int n) {int k=2;while (k<=n) {if (k==n) {System.out.println(k);break;}else {if (n%k==0) {System.out.println(k+"*");n=n/k;}else {k++;}}}}}

八.修改参数的 编码

public class Web {public static void main(String[] args) throws UnsupportedEncodingException {String userName="123";String userName1=new String(userName.getBytes("iso-8851"), "utf-8");}}

九.

//在一个十位的随机有序数组中插入一个数，重新排序public class Test6 {public static void main(String[] args) {int a[]= {2,3,7,10};System.out.println("请输入一个数字");Scanner scanner=new Scanner(System.in);int num=scanner.nextInt();//步骤一:先找到num插入到数组a中的位置int index=0;for (; index < a.length-1; index++) {if (num>=a[a.length-1]) {//若为最大则插入到最后一位index=a.length;break;}if (num<a[index]) {System.out.println("位置为"+index);break;}}//步骤二:新建数组b,将插入位置之前的赋值给新数组int b[]=new int[a.length+1];for (int i = 0; i <=index; i++) {if (i==index) {b[i]=num;}else {b[i]=a[i];}}//步骤三:将插入位置之后的赋值给新数组for (int i = index; i < a.length; i++) {b[i+1]=a[i];}for (int x : b) {System.out.println(x);}}}

十.递归遍历文件夹

public class Test7 {public static void main(String[] args) {File file=new File("E:/");//要遍历的文件夹if (file!=null) {list(file);}}static void list(File f) {if (f.isDirectory()) {File[] files=f.listFiles();for (File file : files) {list(file);//递归调用}}else {System.out.println(f);}}}

十一:换行输出

public class Test8 {public static void main(String[] args) {    Scanner scanner=new Scanner(System.in);    System.out.print("请输入起始值  ");    int start_num=scanner.nextInt();    System.out.print("请输入终点值  ");    int end_num=scanner.nextInt();    System.out.print("请输入一行显示多少个数  ");    int n=scanner.nextInt();    fun( start_num,end_num, n );}public static void fun(int start_num,int end_num,int n ){for (int i = start_num; i <= end_num; i++) {System.out.print(i + " ");if (i % n != 0)continue;System.out.println("");// 当数字一行不整除的时候不换行，持续打入，// 当整除的时候,换行并且跳出到外循环继续}}}

十二:打印三角形

public class Test9 {public static void main(String[] args) {// TODO Auto-generated method stubfor(int i=1;i<=5;i++){for(int j=1;j<=5-i;j++){System.out.print(" ");}for(int m=1;m<=i*2-1;m++){System.out.print("*");}System.out.println();}}}

public class Test10 {public static void main(String[] args) { for (int i = 0; i<5; i++ )       {           for (int j = 0; j<i; j++)           {               System.out.print(" ");           }           for (int k = 0; k<2*(5-i)-1; k++)           {               System.out.print("*");           }           System.out.println();  }}}

十三:判断是否是素数

就是在所有比 1 大的整数中,除了 1 和它本身以外,不再有别的 约数,这种整数叫做质数,质数又叫做素数

  求1到100之间的素数

public class Test11 {public static void main(String[] args) {int count = 0;//统计素数的个数for (int i = 0; i <100; i++) {boolean b=false;//定义一个标识符for (int j = 2; j <Math.sqrt(i); j++) {if (i%j==0) {b=false;//有除得尽的代表不是素数所以为falsebreak;}else {b=true;}}if (b==true) {count++;System.out.println(i);}}System.out.println("素数的个数是"+count);}}

十四.去除数组中的重复元素

//将数组中相同的元素去除public class Test12 {public static void main(String[] args) {int a[]= {1,2,5,5,3,3,10};int count=0;//统计重复元素的个数List list=new  ArrayList<>();for (int i = 0; i < a.length-1; i++) {list.add(a[i]);}Set set=new HashSet<>(list);Object a1[]=set.toArray();for (int i = 0; i < a1.length; i++) {System.out.println(a1[i]);}}}

十五.位数反转

public class Test13 {public static void main(String[] args) {//1234===>4321System.out.println("请输入一个整数");Scanner scanner=new Scanner(System.in);int num=scanner.nextInt();System.out.println("反转后的数是");int result = 0;while (true) {int n = num % 10;result = result * 10 + n;num = num / 10;if (num == 0) {break;}}System.out.println(result);}}

public class Test14 {public static void main(String[] args) {String a="我爱中国";//步骤一:将字符串转为字符数组char b[]=a.toCharArray();//步骤二.将数组转为集合.利用集合类的工具进行反转List list=new ArrayList<>();for (int i = 0; i < b.length; i++) {list.add(b[i]);}Collections.reverse(list);System.out.println(list.indexOf(1));StringBuffer stb=new StringBuffer();for (int i = 0; i < b.length; i++) {stb.append(list.get(i));}System.out.println(stb);}}

十六.查找字符串中某字符串的个数

public class Test16 {public static void main(String[] args) {String string="abcabc123ababab";String str="ab";int count=0;while (string.contains(str)) {count++;string=string.substring(string.indexOf(str)+str.length());}System.out.println(count);}}

十七。如何求出某个字符里各个字符的个数

import java.util.ArrayList;import java.util.Collections;import java.util.HashSet;import java.util.Iterator;import java.util.Set;import java.util.TreeSet;import javax.swing.plaf.basic.BasicTreeUI.TreeCancelEditingAction;//题目:一个字符串中可能包含a~z中的 多个字符,如有重复,如String data="aavzjksdhdksahdkas"//求出现次数最多的那个字母及次数，如有多个重复则都求出public class Test1 {public static void main(String[] args) {String data="aaacccdddddooffggasdae";char[] a=data.toCharArray();//第一步:将字符串转为字符数组ArrayList<String> list=new ArrayList<>();//创建一个list集合TreeSet<String> set=new TreeSet<>();//创建一个set集合int num=0;for (int i = 0; i < a.length; i++) {list.add(String.valueOf(a[i]));set.add(String.valueOf(a[i]));}Collections.sort(list);for (int i = 0; i < list.size()-1; i++) {while (!list.get(i).equals(list.get(i+1))) {System.out.println(list.get(i)+"的个数"+(i+1-list.indexOf(list.get(i))));break;}}}}