Java入门练习题及其答案第一弹

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• Java入门练习题及其答案第一弹
• • 素数打印
  • 乘法口诀表
  • 大公约数
  • 水仙花数
  • 二进制中1的个数
  • 二进制奇偶数位

素数打印

1. 只能被1和自己整除

import java.util.Scanner;

public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
    //这里通过if去判断1 或者是负数的情况
        int i = 2;
        for (; i< n ; i++) {if(n % i == 0){//System.out.println(i + "不是素数!");
                break;
            }
        }
        if(i == n) {System.out.println(n + "是素数！");
        }
    }

2. n ->a*b

   2 – n/2

public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int i = 2;
        for (; i<= n/2 ; i++) {if(n % i == 0){break;
            }
        }
        if(i >n/2) {System.out.println(n + "是素数！");
        }
    }

3. 一定有一个数字小于等于根号n

   public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
           int n = scanner.nextInt();
           int i = 2;
           for (; i<= Math.sqrt(n) ; i++) {if(n % i == 0){break;
               }
           }
           if(i >Math.sqrt(n)) {System.out.println(n + "是素数！");
           }
       }

   4. 打印1 – n间所有素数

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int j = 1; j<= n; j++) {int i = 2;
            for (; i<= Math.sqrt(j); i++) {if (j % i == 0) {break;
                }
            }
            if (i >Math.sqrt(j)) {System.out.println(j);
            }
        }
    }
}

乘法口诀表

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for (int i = 1; i<= n; i++) {for (int j = 1; j<= i; j++) {System.out.print(i + "*" + j + "=" + i*j + " ");
            }
            System.out.println();
        }
    }
}

大公约数

辗转相除法：除到余数为0

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        int c = a % b;
        while(c != 0) {a = b;
            b = c;
            c = a % b;
        }
        System.out.println(b);
    }
}

水仙花数

0~999间所有水仙花数(三位自逆数)

思路：

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();//999999
        for (int i = 0; i< n; i++) {//判断i是几位数 求得每个位上是几 加起来== 原来的数字
            int count = 0;
            int tmp = i;
            while(tmp != 0) {count++;
                tmp = tmp / 10;
            }
            //tmp=0  i不变
            tmp = i;
            int sum = 0;
            while(tmp != 0) {sum += Math.pow(tmp % 10,count);
                tmp /= 10;
            }
            if(sum == i) {System.out.println(i);
            }
        }
    }
}

二进制中1的个数

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        for (int i = 0; i< 32; i++) {if(((n >>>i) & 1) != 0) {count++;
            }
        }
        System.out.println(count);
    }
}

另解

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        while(n != 0) {if((n & 1) != 0) {count++;
            }
            n = n >>>1;
        }
        System.out.println(count);
    }
}

更高效的思路

import java.util.Scanner;
public class Measure {public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int count = 0;
        while(n != 0) {n = n & (n-1);
            count++;
        }
        System.out.println(count);
    }
}

二进制奇偶数位

public static void main(String[] args) {Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        //偶数位
        for (int i = 31; i >= 1; i -= 2) {System.out.print(((n >>i) & 1) + " ");
        }
        System.out.println();
        for (int i = 30; i >= 0 ; i -= 2) {System.out.print(((n >>i) & 1) + " ");
        }
    }

结果：

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