Count and Say Problem in DSA Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the base case for the Count and Say sequence?

The base case is the first term, which is "1". This is the starting point for generating the sequence.

beginner

Explain how the term "21" is generated in the Count and Say sequence.

The term "21" means "one 2, then one 1". It comes from reading the previous term "11" as two ones, which is "21".

beginner

What is the main idea behind the Count and Say algorithm?

The main idea is to read the previous term aloud by counting consecutive digits and saying the count followed by the digit.

intermediate

In Java, which data structure is commonly used to build the next term in the Count and Say sequence efficiently?

A StringBuilder is commonly used to efficiently build the next term by appending counts and digits.

intermediate

Why is it important to reset the count when the digit changes while generating the Count and Say sequence?

Because each group of identical digits must be counted separately, resetting the count ensures accurate grouping and correct next term generation.

What is the 4th term in the Count and Say sequence?

A 312211

B 111221

C 21

D 1211

The sequence starts as 1, 11, 21, 1211. The 4th term is "1211".

In the Count and Say sequence, how do you describe the term "111221"?

A "Three 1s, two 2s, one 1"

B "One 1, one 2, two 1s"

C "Two 1s, two 2s, one 1"

D "One 3, one 1, two 2s"

The term "111221" is read as "three 1s, two 2s, one 1".

Which Java method is best to use for efficient string concatenation in Count and Say?

A StringBuilder.append()

B String.concat()

C StringBuffer.toString()

D String.replace()

StringBuilder.append() is efficient for building strings in loops.

What should you do when the current digit differs from the previous digit in Count and Say?

A Continue counting without reset

B Reset count and start counting the new digit

C Ignore the new digit

D Stop the sequence

Resetting count is necessary to count the new digit group correctly.

What is the output of the Count and Say sequence for n=1?

A 21

B 11

C 1

D 1211

The first term (n=1) is always "1".

Describe the steps to generate the nth term in the Count and Say sequence.

Think about how you read aloud the digits and count them.

Explain why the Count and Say problem is a good example to practice string manipulation and loops.

Focus on how the sequence is formed step-by-step.

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