Count and Say Problem in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the first term of the count-and-say sequence.

DSA Java

String result = [1];

Drag options to blanks, or click blank then click option'

A"1"

B"0"

C"10"

D"11"

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer 1 instead of string "1".

Starting with "0" or other digits.

The count-and-say sequence always starts with "1" as the first term.

2fill in blank

medium

Complete the code to iterate from the second term up to n in the count-and-say sequence.

DSA Java

for (int i = 2; i <= [1]; i++) {

Drag options to blanks, or click blank then click option'

Acount

Bi

Cresult

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using the loop variable i as the upper bound.

Using a fixed number instead of n.

We loop from 2 to n to build each term of the sequence.

3fill in blank

hard

Fix the error in the code that counts consecutive characters in the current term.

DSA Java

if (currentChar == [1].charAt(j)) {

Drag options to blanks, or click blank then click option'

Aresult

Bprev

Ctemp

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with result.charAt(j) which is the next term being built.

Using an undefined variable.

We compare with the character at position j in the current term stored in temp.

4fill in blank

hard

Fill both blanks to append the count and character to the next term string builder.

DSA Java

sb.append([1]).append([2]);

Drag options to blanks, or click blank then click option'

Acount

BcurrentChar

Ci

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Appending the character before the count.

Appending wrong variables like indices.

We append the count of characters and then the character itself to build the next term.

5fill in blank

hard

Fill all three blanks to update the current character, reset count, and convert StringBuilder to string.

DSA Java

currentChar = [1];
count = [2];
result = [3].toString();

Drag options to blanks, or click blank then click option'

Atemp.charAt(j)

B1

Csb

Dresult

Attempts:

3 left

💡 Hint

Common Mistakes

Not resetting count to 1.

Assigning result to wrong variable.

Not updating currentChar correctly.

After appending, update currentChar to the new character, reset count to 1, and convert sb to string for the next iteration.

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