1.完数

/*** 如果一个数等于其所有因子之和,我们就称这个数为"完数",* 例如 6的因子为1,2,3  6=1+2+3   * 6就是一个完数.请编程打印出1000以内所有的完数*/
class wanshu {public static void main(String[] args) {int i,j;for(i=1;i<=1000;i++){   //判断一千以内的完数int sum=0;for(j=1;j<i;j++){if(i%j==0)       // 判断i的因子sum=sum+j;}if(sum==i)          // 判断该数因子的和是否为完数System.out.print(i+"\t");//6 28 496}}
}

2.水仙花数

/*** 打印出所有的"水仙花数"，* 所谓"水仙花数"是指一个三位数，其各位数字立方和等于该数本身。* 例如：153 是一个"水仙花数"，因为153=1³＋5³＋3³。*/class NarciNum {public static void main(String [] args){for(int z = 100; z>=100&&z<1000; z++){int i=0;i=z/100;int j=z/10%10;int x=z/1%10;;if(j*j*j+i*i*i+x*x*x==z)System.out.println(z+"是水仙花数");}}
}

3.素数

//列出1-1000的质数/素数class PrimeNumber {public static void main(String[] args) {for(int i  = 2 ; i <= 1000 ; i++){boolean flag = true;for(int j = 2 ; j < i ; j++){if(i % j == 0){flag = false;break;}}if(flag){System.out.println(i);}}}
}

4.阿姆斯特朗数

/*** 如果一个n位正整数等于其各位数字的n次方之和,则称该数为阿姆斯特朗数。* 例如1^3 + 5^3 + 3^3 = 153* 当n=3时，又称水仙花数，特指一种三位数，其各个数之立方和等于该数。* 水仙花数共有4个，分别为：153、370、371、407*/
class Test{public static void main(String[] args){for(int i = 1 ;i<= 999 ;i++) show(i);}public static void show(int i){ int sum = 0;int temp = i;String str = i+"";while(temp != 0){int num = temp % 10;sum += Math.pow(num,str.length());temp /= 10;}if(sum == i) System.out.println(i);}
}

5.斐波那契数列

//斐波那契数列class Fib {public static void main(String args[]) {for (int i = 1; i <= 20; i++)System.out.println(f(i));}public static int f(int x) {if (x == 1||x == 2)return 1;elsereturn f(x - 1) + f(x - 2);}
}

6.最大公约数和最小公倍数

import java.util.Scanner;class GCDLCM {// 最大公约数public static int get_gcd(int n1, int n2) {int gcd = 0;if (n1 < n2) {// 交换n1、n2的值n1 = n1 + n2;n2 = n1 - n2;n1 = n1 - n2;}if (n1 % n2 == 0) {gcd = n2;}while (n1 % n2 > 0) {n1 = n1 % n2;if (n1 < n2) {n1 = n1 + n2;n2 = n1 - n2;n1 = n1 - n2;}if (n1 % n2 == 0) {gcd = n2;}}return gcd;}// 最小公倍数public static int get_lcm(int n1, int n2) {return n1 * n2 / get_gcd(n1, n2);}public static void main(String[] args) {Scanner input = new Scanner(System.in);System.out.print("请输入第一个整数：");int n1 = input.nextInt();System.out.print("请输入第二个整数：");int n2 = input.nextInt();System.out.println("最大公约数：" + "(" + n1 + "," + n2 + ")" + "=" + get_gcd(n1, n2));System.out.println("最小公倍数：" + "[" + n1 + "," + n2 + "]" + "=" + get_lcm(n1, n2));}
}

7.字符统计

/*将字符串以a(字母)=>2(个数)存入Map集合框架中思路:1.将字符串转换成字符数组.2.定义一个Map集合,然后对字符数组进行遍历,如果Map集合中没有该元素就将该元素存入Map集合中,并定义一个计数器,将次数存入Map中,来达到目的 */
import java.util.*;
class CharMapDemo{public static void main(String[] args){String str2 = "";Scanner scan = new Scanner(System.in);System.out.println("请输入字符串：");if (scan.hasNextLine()) {str2 = scan.nextLine();}scan.close();TreeMap<Character,Integer> tm = Pross(str2);System.out.println(tm);}public static TreeMap<Character,Integer> Pross(String str){char[] charArray = str.toCharArray();TreeMap<Character,Integer> tm = new TreeMap<Character,Integer>();for (int x = 0; x < charArray.length ;x++ ){if(!tm.containsKey(charArray[x])){tm.put(charArray[x],1);}else{int count = tm.get(charArray[x])+1;tm.put(charArray[x],count);}}return tm;}
}

8.回文判断

import java.util.*;
class HuiWen{public static void main(String[] args){String str = "abac";if(judge(str)){System.out.println("true");}else{System.out.println("false");}  }public static boolean judge (String str) {int len = str.length();for(int i = 0 ; i < len/2 ;i++) if(str.charAt(i)!=str.charAt(len-1-i)) return false;return true;}}

import java.util.*;
/***回文数*/public class Solution {/*** * @param x int整型 * @return bool布尔型*/public boolean isPalindrome (int x) {// write code here// 边界判断if (x < 0) return false;int div = 1;//while (x / div >= 10) div *= 10;// 循环停止while (x > 0) {int left = x / div;int right = x % 10;// 如果不同则表示不是回文if (left != right) return false;x = (x % div) / 10;div /= 100;}return true;}
}

9.归并排序

//归并排序class MergeSort {//两路归并算法，两个排好序的子序列合并为一个子序列public void merge(int []a,int left,int mid,int right){int []tmp=new int[a.length];//辅助数组int p1=left,p2=mid+1,k=left;//p1、p2是检测指针，k是存放指针while(p1<=mid && p2<=right){if(a[p1]<=a[p2])tmp[k++]=a[p1++];elsetmp[k++]=a[p2++];}while(p1<=mid) tmp[k++]=a[p1++];//如果第一个序列未检测完，直接将后面所有元素加到合并的序列中while(p2<=right) tmp[k++]=a[p2++];//同上//复制回原素组for (int i = left; i <=right; i++) a[i]=tmp[i];}public void mergeSort(int [] a,int start,int end){if(start<end){//当子序列中只有一个元素时结束递归int mid=(start+end)/2;//划分子序列mergeSort(a, start, mid);//对左侧子序列进行递归排序mergeSort(a, mid+1, end);//对右侧子序列进行递归排序merge(a, start, mid, end);//合并}}public static void main(String[] args){int[] a = { 49, 38, 65, 97, 76, 13, 27, 50 };MergeSort merge = new MergeSort();merge.mergeSort(a, 0, a.length-1);System.out.println("排好序的数组：");for (int e : a)System.out.print(e+" ");}
}

10.冒泡排序

class bubble_sort {public static void main(String[] args) {//冒泡排序算法int[] numbers=new int[]{1,5,8,2,3,9,4};//需进行length-1次冒泡for(int i=0;i<numbers.length-1;i++){for(int j=0;j<numbers.length-1-i;j++){if(numbers[j]>numbers[j+1]){int temp=numbers[j];numbers[j]=numbers[j+1];numbers[j+1]=temp;}}}System.out.println("从小到大排序后的结果是:");for(int i=0;i<numbers.length;i++)System.out.print(numbers[i]+" ");}
}

11.选择排序

//选择排序
class SelectionSort {public static void main(String[] args) {int[] arr={1,3,2,45,65,33,12};System.out.println("交换之前：");for(int num:arr){System.out.print(num+" ");}        //选择排序的优化for(int i = 0; i < arr.length - 1; i++) {// 做第i趟排序int k = i;for(int j = k + 1; j < arr.length; j++){// 选最小的记录if(arr[j] < arr[k]){ k = j; //记下目前找到的最小值所在的位置}}//在内层循环结束，也就是找到本轮循环的最小的数以后，再进行交换if(i != k){  //交换a[i]和a[k]int temp = arr[i];arr[i] = arr[k];arr[k] = temp;}    }System.out.println();System.out.println("交换后：");for(int num:arr){System.out.print(num+" ");}}}

12.插入排序

class Insert
{public static void main(String[] args){int[] ins = {2,3,5,1,23,6,78,34};int[] ins2 = sort(ins);for(int in: ins2){System.out.println(in);}}public static int[] sort(int[] ins){for(int i=1; i<ins.length; i++){int temp = ins[i];//保存每次需要插入的那个数int j;for(j=i; j>0&&ins[j-1]>temp; j--){//这个较上面有一定的优化ins[j] = ins[j-1];//吧大于需要插入的数往后移动。最后不大于temp的数就空出来j}ins[j] = temp;//将需要插入的数放入这个位置}return ins;}
}

13.快速排序

import java.util.Arrays;
class QuickSort{public static void main (String[] args) {int[] arr = new int[] {7,1,9,2,5,3,12,4};quickSort(arr);System.out.println(Arrays.toString(arr));}/*** 快速排序* @param array*/public static void quickSort(int[] array) {int len;if(array == null|| (len = array.length) == 0|| len == 1) {return ;}sort(array, 0, len - 1);}/*** 快排核心算法，递归实现* @param array* @param left* @param right*/public static void sort(int[] array, int left, int right) {if(left > right) {return;}// base中存放基准数int base = array[left];int i = left, j = right;while(i != j) {// 顺序很重要，先从右边开始往左找，直到找到比base值小的数while(array[j] >= base && i < j) {j--;}// 再从左往右边找，直到找到比base值大的数while(array[i] <= base && i < j) {i++;}// 上面的循环结束表示找到了位置或者(i>=j)了，交换两个数在数组中的位置if(i < j) {int tmp = array[i];array[i] = array[j];array[j] = tmp;}}// 将基准数放到中间的位置（基准数归位）array[left] = array[i];array[i] = base;// 递归，继续向基准的左右两边执行和上面同样的操作// i的索引处为上面已确定好的基准值的位置，无需再处理sort(array, left, i - 1);sort(array, i + 1, right);}
}

14.二分查找

class binarySearch {public static void main(String[] args) {int[] array ={1,2,3,4,6,7,8,9,10};System.out.println( binSearch_2(3,array,0,array.length-1));System.out.println( binSearch_2(5,array,0,array.length-1));}/**循环实现二分算法*/public static int binSearch_1(int key, int[] array) {int low = 0; //第一个下标int high = array.length - 1;//最后一个下标int middle = 0;//防越界if (key < array[low] || key > array[high] || low > high) {return -1;}while (low <= high) {middle = (low + high) / 2;if (middle == key) {return array[middle];} else if (middle < key) {low = middle + 1;} else {high = middle - 1;}}return -1;}/**递归实现二分算法*/public static int binSearch_2(int key,int[] array,int low,int high){//防越界if (key < array[low] || key > array[high] || low > high) {return -1;}int middle = (low+high)/2;if(array[middle]>key){//大于关键字return  binSearch_2(key,array,low,middle-1);}else if(array[middle]<key){//小于关键字return binSearch_2(key,array,middle+1,high);}else{return array[middle];}}
}

15.二叉树先序遍历

//前序遍历的递归实现与非递归实现
import java.util.Stack;
class Test {public static void main(String[] args){TreeNode[] node = new TreeNode[10];//以数组形式生成一棵完全二叉树for(int i = 0; i < 10; i++)node[i] = new TreeNode(i);for(int i = 0; i < 10; i++){if(i*2+1 < 10)node[i].left = node[i*2+1];if(i*2+2 < 10)node[i].right = node[i*2+2];}preOrderRe(node[0]);}public static void preOrderRe(TreeNode biTree){//递归实现System.out.println(biTree.value);TreeNode leftTree = biTree.left;if(leftTree != null){preOrderRe(leftTree);}TreeNode rightTree = biTree.right;if(rightTree != null){preOrderRe(rightTree);}}public static void preOrder(TreeNode biTree){//非递归实现Stack<TreeNode> stack = new Stack<TreeNode>();while(biTree != null || !stack.isEmpty()){while(biTree != null){System.out.println(biTree.value);stack.push(biTree);biTree = biTree.left;}if(!stack.isEmpty()){biTree = stack.pop();biTree = biTree.right;}}}
}class TreeNode{//节点结构int value;TreeNode left;TreeNode right;TreeNode(int value){this.value = value;}
}