Count and Say Problem in DSA Java - Build from Scratch

Count and Say Problem

📖 Scenario: Imagine you have a sequence of numbers where each term describes the previous term by counting the number of digits in groups. This is called the "Count and Say" sequence. For example, starting with "1", the next term describes it as "one 1" which is "11", then "two 1s" which is "21", and so on.

🎯 Goal: You will build a Java program that generates the nth term of the Count and Say sequence. This helps understand how to process strings and use loops to build new sequences step-by-step.

📋 What You'll Learn

Create a variable to hold the starting string "1"

Create a variable n to represent which term to generate

Use a loop to build the sequence up to the nth term

Print the final term after generating it

💡 Why This Matters

🌍 Real World

The Count and Say sequence is a simple example of run-length encoding, a technique used in data compression to reduce repeated data.

💼 Career

Understanding string processing and loops is essential for software development, especially in tasks like parsing, data compression, and algorithm design.

1

Create the starting term

Create a String variable called term and set it to the value "1".

DSA Java

# Your code here
// Create the starting term string

Hint

The first term in the Count and Say sequence is always "1".

2

Set the term number to generate

Add an integer variable called n and set it to 5 to represent the term number you want to generate.

DSA Java

public class CountAndSay {
    public static void main(String[] args) {
        String term = "1";
        // Set the term number to generate
        
    }
}

Hint

Choose a number for n to find that term in the sequence.

3

Generate the Count and Say sequence

Write a for loop that runs from 2 to n inclusive. Inside the loop, create a new StringBuilder called nextTerm. Use a while loop with an index i to count consecutive repeating characters in term. Append the count and the character to nextTerm. After processing, set term to nextTerm.toString().

DSA Java

public class CountAndSay {
    public static void main(String[] args) {
        String term = "1";
        int n = 5;
        // Generate the sequence up to n
        
    }
}

Hint

Use nested loops: outer for loop for terms, inner while loop to count characters.

4

Print the final term

Write a System.out.println(term); statement to display the final generated term after the loop.

DSA Java

public class CountAndSay {
    public static void main(String[] args) {
        String term = "1";
        int n = 5;
        for (int count = 2; count <= n; count++) {
            StringBuilder nextTerm = new StringBuilder();
            int i = 0;
            while (i < term.length()) {
                char currentChar = term.charAt(i);
                int charCount = 1;
                while (i + 1 < term.length() && term.charAt(i) == term.charAt(i + 1)) {
                    charCount++;
                    i++;
                }
                nextTerm.append(charCount).append(currentChar);
                i++;
            }
            term = nextTerm.toString();
        }
        // Print the final term
        
    }
}

Hint

The 5th term in the Count and Say sequence starting from "1" is "111221".

Practice

1. What does the Count and Say sequence represent in simple terms?

easy

A. It reverses the digits of a number.

B. It describes the count of digits seen in the previous term.

C. It sorts numbers in ascending order.

D. It adds two numbers repeatedly.

Solution

1. Step 1: Understand the sequence definition

   The Count and Say sequence describes each term by counting the number of digits appearing consecutively in the previous term.

2. Step 2: Relate to example

   For example, starting with "1", the next term is "11" because there is one '1'. Then "21" because there are two '1's, and so on.

3. Final Answer:

   It describes the count of digits seen in the previous term. -> Option B

4. Quick Check:

   Count and Say = Count digits [OK]

Hint: Remember: count digits, then say them aloud [OK]

Common Mistakes:

• Thinking it sorts digits
• Confusing with reversing digits
• Assuming it adds numbers

2. Which of the following Java method signatures is correct for a function that returns the nth term of the Count and Say sequence?

easy

A. public String countAndSay(int n)

B. public int countAndSay(String n)

C. private void countAndSay(int n)

D. public String countAndSay()

Solution

1. Step 1: Identify correct return type and parameters

   The function should return a String representing the sequence term and take an integer n as input.

2. Step 2: Check method signature options

   public String countAndSay(int n) matches this: public String countAndSay(int n). Others either have wrong return type, parameters, or access modifiers.

3. Final Answer:

   public String countAndSay(int n) -> Option A

4. Quick Check:

   Return String, input int n [OK]

Hint: Return String, input int n for Count and Say method [OK]

Common Mistakes:

• Using void return type
• Wrong parameter type
• Missing input parameter

3. What is the output of the following Java code snippet?

System.out.println(countAndSay(4));

Assuming countAndSay is correctly implemented for the Count and Say sequence.

medium

A. "111221"

B. "21"

C. "1211"

D. "312211"

Solution

1. Step 1: Trace the sequence terms up to n=4

   Term 1: "1"
   Term 2: "11" (one 1)
   Term 3: "21" (two 1s)
   Term 4: "1211" (one 2, two 1s)

2. Step 2: Confirm output for n=4

   The output for countAndSay(4) is "1211".

3. Final Answer:

   "1211" -> Option C

4. Quick Check:

   Term 4 = "1211" [OK]

Hint: Write terms stepwise to find nth output [OK]

Common Mistakes:

• Confusing term 3 and 4
• Skipping counting digits correctly
• Mixing digits order

4. Identify the error in this Java code snippet for Count and Say:

public String countAndSay(int n) {
  if (n == 1) return "1";
  String prev = countAndSay(n - 1);
  String result = "";
  int count = 1;
  for (int i = 1; i < prev.length(); i++) {
    if (prev.charAt(i) == prev.charAt(i - 1)) {
      count++;
    } else {
      result += count + prev.charAt(i);
      count = 1;
    }
  }
  return result;
}

medium

A. The method should return an integer, not a String.

B. The code appends count before the correct character in else block.

C. The base case for n == 1 is missing.

D. The loop does not handle the last group of characters after the loop ends.

Solution

1. Step 1: Analyze loop behavior

   The loop processes characters but does not append the last counted group after finishing the loop.

2. Step 2: Identify missing step

   After the loop, the last counted group must be appended to the result to complete the sequence term.

3. Final Answer:

   The loop does not handle the last group of characters after the loop ends. -> Option D

4. Quick Check:

   Missing last group append causes incomplete output [OK]

Hint: Always append last counted group after loop ends [OK]

Common Mistakes:

• Forgetting to append last group
• Appending wrong character
• Incorrect base case

5. Given the Count and Say sequence starting with "1", what is the 6th term in the sequence?

hard

A. "312211"

B. "13112221"

C. "1113213211"

D. "21133112"

Solution

1. Step 1: Generate terms up to 6

   Term 1: "1"
   Term 2: "11" (one 1)
   Term 3: "21" (two 1s)
   Term 4: "1211" (one 2, two 1s)
   Term 5: "111221" (three 1s, two 2s, one 1)
   Term 6: "312211" (three 1s, two 2s, one 1)

2. Step 2: Confirm 6th term

   The 6th term is "312211".

3. Final Answer:

   "312211" -> Option A

4. Quick Check:

   Term 6 = "312211" [OK]

Hint: Build terms stepwise to avoid mistakes [OK]

Common Mistakes:

• Skipping terms or mixing order
• Miscounting digits
• Confusing digits in groups