Hello! Today, I solved algorithm problems through a coding test course using Java. In particular, I will introduce a problem related to ‘hacking’ and explain the process of solving it step by step. This course is aimed at those who primarily use the Java language and includes content designed to enhance algorithm problem-solving skills.
Problem Description
Problem Title: Hacker’s Password
The hacker is trying to discover the password to break into a small company’s server. Luckily, the hacker knows a series of previous passwords. These passwords consist of different alphabet characters. The hacker must identify the ‘most frequently used alphabet’ to guess the password.
Problem Definition
Input: - N (1 ≤ N ≤ 1000): Number of passwords - Password: Maximum length of 50, consisting only of lowercase alphabet letters. Output: Print the most frequently used alphabet and its count in the format "alphabet: count".
Example Input
5 abcde fghij klmno abcfg hijkl
Example Output
a: 2 b: 2 c: 2 f: 2 g: 2 h: 2 i: 2 j: 2 k: 2 l: 2 m: 1 n: 1 o: 1
Problem Solving Approach
To solve this problem, I considered the following approach.
Step 1: Understand and Analyze the Problem
The given problem is to count the frequency of alphabets appearing in each password and print the most frequently occurring alphabet. Considering the number of inputs and the characteristics of each password, this problem is algorithmically suitable for the Counting method. By counting, we can implement a structure to count the frequency of each alphabet.
Step 2: Design Data Structure
We will use a HashMap
to store the frequency of alphabets. The HashMap stores data as key-value pairs, where the key is the alphabet character (‘a’~’z’) and the value is the frequency of that alphabet. This structure allows us to traverse through each password and calculate the frequency.
Step 3: Design Algorithm
1. Create a HashMap to store the count of alphabets. 2. Traverse through the given passwords. 3. For each password, traverse again and count the alphabets. 4. Output the results in the following format: "alphabet: count".
Java Code Implementation
Now that the algorithm is established, let’s implement it in Java code.
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
public class HackerPassword {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
scanner.nextLine(); // Clear buffer
Map frequencyMap = new HashMap<>();
// Input passwords and counting
for (int i = 0; i < n; i++) {
String password = scanner.nextLine();
for (char c : password.toCharArray()) {
frequencyMap.put(c, frequencyMap.getOrDefault(c, 0) + 1);
}
}
// Output results
for (Map.Entry entry : frequencyMap.entrySet()) {
System.out.println(entry.getKey() + ": " + entry.getValue());
}
}
}
Code Explanation
Let’s look at the code step by step:
- Input Handling: First, it uses the Scanner class to input the number of passwords
n
, and then reads the passwords. - HashMap Initialization: Initializes the HashMap to record the frequency of each alphabet.
- Frequency Counting: Uses a nested for loop to take each password as a string and count each character in the HashMap. It uses the
getOrDefault
method to set a default value of 0 for counting. - Output Results: Finally, it traverses the HashMap to print the alphabet and its frequency.
Testing and Validation
The algorithm and code should be validated with various test cases. It should be checked whether the frequency of each alphabet is correctly output even when multiple passwords are input according to the given conditions. For example:
Test Case 1
Input: 3 aaa bbb c Output: a: 3 b: 3 c: 1
Test Case 2
Input: 4 abcd efgh ijkl mnop Output: a: 1 b: 1 c: 1 d: 1 e: 1 f: 1 g: 1 h: 1 i: 1 j: 1 k: 1 l: 1 m: 1 n: 1 o: 1 p: 1
By comparing the expected results with the actual outputs for each test case, we can validate the accuracy of the code.
Conclusion
In today’s lecture, we addressed an algorithm problem of counting the frequency of alphabets to hack a password. Such algorithm problems frequently appear in coding tests, and various variations exist. I hope this course has taught you how to approach algorithm problems and helped deepen your understanding of the Java programming language.
Next time, I plan to introduce more complex algorithm problems and discuss optimization techniques as well. Thank you for reading until the end!