Classic Java Algorithm Exercises with Sample Solutions

Even when doing web development, you often encounter algorithmic challenges; this article extracts several classic practice algorithms and provides reference solutions in Java.

Program 1 – Fibonacci Rabbit Problem

Problem: A pair of rabbits reproduces each month after the third month, forming a Fibonacci sequence. Compute the total number of rabbits each month.

public class test01 {
    public static void main(String[] args) {
        int f1=1,f2=1,f;
        int M=30;
        System.out.println(1);
        System.out.println(2);
        for(int i=3;i<M;i++) {
            f=f2;
            f2=f1+f2;
            f1=f;
            System.out.println(f2);
        }
    }
}

Program 2 – Prime Numbers Between 101 and 200

Problem: Count and output all prime numbers in the range 101‑200.

public class test02 {
    public static void main(String[] args) {
        int count=0;
        for(int i=101;i<200;i+=2) {
            boolean flag=true;
            for(int j=2;j<=Math.sqrt(i);j++) {
                if(i%j==0) {
                    flag=false;
                    break;
                }
            }
            if(flag==true) {
                count++;
                System.out.println(i);
            }
        }
        System.out.println(count);
    }
}

Program 3 – Armstrong (Narcissistic) Numbers

Problem: Print all three‑digit numbers whose sum of cubes of digits equals the number itself (e.g., 153).

public class test03 {
    public static void main(String[] args) {
        int a,b,c;
        for(int i=101;i<1000;i++) {
            a=i%10;
            b=i/10%10;
            c=i/100;
            if(a*a*a+b*b*b+c*c*c==i)
                System.out.println(i);
        }
    }
}

Program 4 – Prime Factorization

Problem: Decompose a positive integer into its prime factors.

import java.util.Scanner;
public class test04 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        int n=input.nextInt();
        int k=2;
        while(n>=k) {
            if(n==k) {
                System.out.println(k);
                break;
            } else if(n%k==0) {
                System.out.println(k);
                n=n/k;
            } else {
                k++;
            }
        }
    }
}

Program 5 – Grade Classification Using Nested Ternary Operator

Problem: Map a score to grades A, B, or C based on thresholds.

import java.util.Scanner;public class test05 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        int score=input.nextInt();
        char grade=score>=90?'A':score>=60?'B':'C';
        System.out.println(grade);
    }
}

Program 6 – GCD and LCM

Problem: Compute the greatest common divisor and least common multiple of two positive integers.

import java.util.Scanner;public class test06 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        int a=input.nextInt();
        int b=input.nextInt();
        test06 test=new test06();
        int i = test.gongyinshu(a, b);
        System.out.println("最小公因数"+i);
        System.out.println("最大公倍数"+a*b/i);
    }
    public int gongyinshu(int a,int b) {
        if(a<b) {
            int t=b;
            b=a;
            a=t;
        }
        while(b!=0) {
            if(a==b)
                return a;
            int x=b;
            b=a%b;
            a=x;
        }
        return a;
    }
}

Program 7 – Character Statistics

Problem: Count letters, spaces, digits, and other characters in an input line.

import java.util.Scanner;public class test07 {
    public static void main(String[] args) {
        int abccount=0,spacecount=0,numcount=0,othercount=0;
        Scanner input=new Scanner(System.in);
        String toString=input.nextLine();
        char[] ch=toString.toCharArray();
        for(int i=0;i<ch.length;i++) {
            if(Character.isLetter(ch[i])) {
                abccount++;
            } else if(Character.isDigit(ch[i])) {
                numcount++;
            } else if(Character.isSpaceChar(ch[i])){
                spacecount++;
            } else {
                othercount++;
            }
        }
        System.out.println(abccount);
        System.out.println(spacecount);
        System.out.println(numcount);
        System.out.println(othercount);
    }
}

Program 8 – Sum of Series a + aa + aaa + …

Problem: Compute the sum of a series where each term appends the digit a (e.g., 2+22+222+…).

import java.util.Scanner;public class test08 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        int a=input.nextInt();
        int n=input.nextInt();
        int sum=0,b=0;
        for(int i=0;i<n;i++) {
            b+=a;
            sum+=b;
            a=a*10;
        }
        System.out.println(sum);
    }
}

Program 9 – Perfect Numbers up to 1000

Problem: Find all numbers equal to the sum of their proper divisors.

public class test09 {
    public static void main(String[] args) {
        for(int i=1;i<=1000;i++) {
            int t = 0;
            for(int j=1;j<=i/2;j++) {
                if(i%j==0) {
                    t+=j;
                }
            }
            if(t==i) {
                System.out.println(i);
            }
        }
    }
}

Program 10 – Falling Ball Distance and Bounce Height

Problem: A ball drops from 100 m, rebounds to half the height each time; compute total distance after 10 contacts and the 10th bounce height.

public class test10 {
    public static void main(String[] args) {
        double h=100;
        double s=100;
        for(int i=1;i<=10;i++) {
            h=h/2;
            s=s+2*h;
        }
        System.out.println(s);
        System.out.println(h);
    }
}

Program 11 – Three‑Digit Permutations from {1,2,3,4}

Problem: Generate all distinct three‑digit numbers without repeated digits using digits 1‑4.

class test11 {
    public static void main(String[] args) {
        int count=0;
        for(int i=1;i<5;i++) {
            for(int j=1;j<5;j++) {
                for(int k=1;k<5;k++) {
                    if(i!=j&&j!=k&&i!=k) {
                        count++;
                        System.out.println(i*100+j*10+k);
                    }
                }
            }
        }
        System.out.println(count);
    }
}

Program 12 – Profit‑Based Bonus Calculation

Problem: Compute bonus based on tiered profit percentages.

test12 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        double x=input.nextDouble();
        double y=0;
        if(x>0&&x<=10) {
            y=x*0.1;
        } else if(x>10&&x<=20) {
            y=10*0.1+(x-10)*0.075;
        } else if(x>20&&x<=40) {
            y=10*0.1+10*0.075+(x-20)*0.05;
        } else if(x>40&&x<=60) {
            y=10*0.1+10*0.075+20*0.05+(x-40)*0.03;
        } else if(x>60&&x<=100) {
            y=10*0.1+10*0.075+20*0.05+20*0.03+(x-60)*0.015;
        } else if(x>100) {
            y=10*0.1+10*0.075+20*0.05+20*0.03+40*0.015+(x-100)*0.01;
        }
        System.out.println(y);
    }
}

Program 13 – Integer Satisfying Two Square Conditions

Problem: Find integer n such that n+100 and n+268 are both perfect squares.

public class test13 {
    public static void main(String[] args) {
        for(int i=-100;i<10000;i++) {
            if(Math.sqrt(i+100)%1==0 && Math.sqrt(i+268)%1==0) {
                System.out.println(i);
            }
        }
    }
}

Program 14 – Day of Year Calculator

Problem: Given a date, determine its ordinal day within the year.

import java.util.*;public class lianxi14 {
    public static void main(String[] args) {
        int year, month, day;
        int days = 0;
        int d = 0;
        int e;
        input fymd = new input();
        do {
            e = 0;
            System.out.print("输入年：");
            year = fymd.input();
            System.out.print("输入月：");
            month = fymd.input();
            System.out.print("输入天：");
            day = fymd.input();
            if(year<0||month<0||month>12||day<0||day>31) {
                System.out.println("输入错误，请重新输入！");
                e=1;
            }
        } while(e==1);
        for(int i=1; i<month; i++) {
            switch (i) {
                case 1:case 3:case 5:case 7:case 8:case 10:case 12:
                    days = 31; break;
                case 4:case 6:case 9:case 11:
                    days = 30; break;
                case 2:
                    if((year%400==0)|| (year%4==0 && year%100!=0)) {
                        days = 29;
                    } else {
                        days = 28;
                    }
                    break;
            }
            d += days;
        }
        System.out.println(year+"-"+month+"-"+day+"是这年的第"+(d+day)+"天。");
    }
}
class input{
    public int input() {
        int value = 0;
        Scanner s = new Scanner(System.in);
        value = s.nextInt();
        return value;
    }
}

Program 15 – Sorting Three Integers

Problem: Output three integers in ascending order.

public class test15 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        int x=input.nextInt();
        int y=input.nextInt();
        int z=input.nextInt();
        int t=0;
        if(x>y) { t=x; x=y; y=t; }
        if(y>z) { t=z; z=y; y=t; }
        if(x>y) { t=x; x=y; y=t; }
        System.out.println(x+""+y+""+z);
    }
}

Program 16 – 9×9 Multiplication Table

Problem: Print the classic 9×9 multiplication table.

public class test16 {
    public static void main(String[] args) {
        for(int i=1;i<10;i++){
            for(int j=1;j<=i;j++) {
                System.out.print(i+"*"+j+"="+i*j);
                System.out.print(" ");
            }
            System.out.println("");
        }
    }
}

Program 17 – Monkey and Peach Problem

Problem: Determine the initial number of peaches given the consumption pattern over ten days.

public class test17 {
    public static void main(String[] args) {
        int x=1;
        for(int i=10;i>1;i--) {
            x=(x+1)*2;
        }
        System.out.println(x);
    }
}

Program 18 – Table Tennis Team Matching

Problem: Find a matching of three players from each team respecting given constraints.

public class test18 {
    public static void main(String[] args) {
        for(char i='x';i<='z';i++) {
            for(char j='x';j<='z';j++) {
                if(i!=j) {
                    for(char k='x';k<='z';k++) {
                        if(i!=k && j!=k) {
                            if(i!='x' && j!='x' && j!='z') {
                                System.out.println("a:"+i+"
b:"+j+"
c:"+k);
                            }
                        }
                    }
                }
            }
        }
    }
}

Program 19 – Diamond Pattern Printing

Problem: Print a symmetric diamond shape using asterisks.

public class lianxi19 {
    public static void main(String[] args) {
        int H = 7, W = 7; // high and width must be equal odd numbers
        for(int i=0; i<(H+1)/2; i++) {
            for(int j=0; j<W/2-i; j++) {
                System.out.print(" ");
            }
            for(int k=1; k<(i+1)*2; k++) {
                System.out.print('*');
            }
            System.out.println();
        }
        for(int i=1; i<=H/2; i++) {
            for(int j=1; j<=i; j++) {
                System.out.print(" ");
            }
            for(int k=1; k<=W-2*i; k++) {
                System.out.print('*');
            }
            System.out.println();
        }
    }
}

Program 20 – Sum of First 20 Terms of a Fraction Sequence

Problem: Compute the sum of the series 2/1, 3/2, 5/3, 8/5, … for the first 20 terms.

public class test20 {
    public static void main(String[] args) {
        double sum=0, ver=2;
        for(int i=1;i<=10;i++) {
            sum+=ver/i;
            ver+=i;
        }
        System.out.println(sum);
    }
}

Program 21 – Sum of Factorials 1! to 20!

Problem: Calculate 1 + 2! + 3! + … + 20!.

public class test21 {
    public static void main(String[] args) {
        long sum=0, ver=1;
        for(int i=1;i<=20;i++) {
            ver=ver*i;
            sum+=ver;
        }
        System.out.println(sum);
    }
}

Program 22 – Recursive Computation of 5!

Problem: Use recursion to compute 5 factorial.

public class test22 {
    public static void main(String[] args) {
        System.out.println(fac(5));
    }
    public static int fac(int i) {
        if(i==1) return 1;
        else {
            return i*fac(i-1);
        }
    }
}

Program 23 – Age Increment Puzzle

Problem: Starting from age 10, each subsequent person is 2 years older; find the fifth person's age.

public class test23 {
    public static void main(String[] args) {
        int age=10;
        for(int i=2;i<=5;i++) {
            age+=2;
        }
        System.out.println(age);
    }
}

Program 24 – Digit Count and Reverse Printing

Problem: For a positive integer (≤5 digits), output its length and the digits in reverse order.

import java.util.Scanner;
public class test24 {
    public static void main(String[] args) {
        Scanner input=new Scanner(System.in);
        String toString=input.nextLine();
        char[] num=toString.toCharArray();
        System.out.println(num.length);
        for(int i=num.length;i>0;i--) {
            System.out.print(num[i-1]);
        }
    }
}

Program 25 – Palindrome Check for a 5‑Digit Number

Problem: Determine whether a five‑digit integer is a palindrome.

import java.util.Scanner;public class test25 {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int numtest=input.nextInt();
        System.out.println(ver(numtest));
    }
    public static boolean ver(int num) {
        if(num<0||(num!=0 && num%10==0))
            return false;
        int ver=0;
        while(num>ver) {
            ver=ver*10+num%10;
            num=num/10;
        }
        return(num==ver||num==ver/10);
    }
}

These examples cover a broad range of fundamental algorithmic concepts useful for interview preparation and programming practice.