Count and Say Problem in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Javascript

let result = "[1]";

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

A2

B1

C"1"

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using number 1 instead of string "1".

Starting with 0 or 2 instead of 1.

The first term of the count and say sequence is the string "1".

2fill in blank

medium

Complete the code to iterate over the characters of the current term.

DSA Javascript

for (let i = 0; i < current.length; [1]) {

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Ai++

Bi--

Ci += 2

Di = 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using i-- which causes infinite loop.

Incrementing by 2 which skips characters.

To iterate over each character, increment i by 1 each loop with i++.

3fill in blank

hard

Fix the error in the code to correctly count consecutive characters.

DSA Javascript

if (current[i] === current[[1]]) {

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Ai + 1

Bi

Ci - 1

Di + 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using i - 1 which looks at previous character.

Using i + 2 which skips one character.

To check the next character, compare current[i] with current[i + 1].

4fill in blank

hard

Fill both blanks to build the next term by appending count and character.

DSA Javascript

nextTerm += count.toString() + [1];
count = [2];

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Acurrent[i]

Bcurrent[i + 1]

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Appending the wrong character like current[i + 1].

Resetting count to 0 instead of 1.

Append the current character and reset count to 1 for the next group.

5fill in blank

hard

Fill all three blanks to complete the count and say function.

DSA Javascript

function countAndSay(n) {
  let result = "1";
  for (let [1] = 1; [1] < [3]; [1]++) {
    let current = result;
    let nextTerm = "";
    let count = 1;
    for (let i = 0; i < current.length; i++) {
      if (current[i] === current[i + 1]) {
        count++;
      } else {
        nextTerm += count.toString() + current[i];
        count = 1;
      }
    }
    result = nextTerm;
  }
  return [2];
}

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

Ai

Bresult

Cn

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong loop variable name.

Returning 'n' or 'count' instead of 'result'.

Use 'i' as loop variable, return 'result', and loop until 'n'.

Practice

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

easy

A. It sorts numbers in ascending order.

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

C. It finds the sum of digits in a number.

D. It reverses the digits of a number.

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: Compare options with definition

   Only It describes the count of digits seen in the previous term. matches this description exactly, others describe unrelated operations.

3. Final Answer:

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

4. Quick Check:

   Count and Say = Counting digits [OK]

Hint: Remember: Count and Say counts digits, not sums or sorts [OK]

Common Mistakes:

• Confusing counting digits with summing digits
• Thinking it sorts or reverses digits
• Assuming it generates random sequences

2. Which of the following is the correct JavaScript syntax to start the Count and Say sequence with the first term as '1'?

easy

A. let term = 1;

B. let term = (1);

C. let term = [1];

D. let term = '1';

Solution

1. Step 1: Identify the data type for the term

   The Count and Say sequence terms are strings representing digits, so the first term should be a string '1'.

2. Step 2: Check options for correct string assignment

   let term = '1'; assigns '1' as a string, while others assign number or array types.

3. Final Answer:

   let term = '1'; -> Option D

4. Quick Check:

   Use string for digits in Count and Say [OK]

Hint: Use quotes for digits as strings in Count and Say [OK]

Common Mistakes:

• Using number type instead of string
• Using array instead of string
• Using parentheses which do nothing here

3. What will be the output of the following JavaScript code snippet?

function countAndSay(n) {
  if (n === 1) return '1';
  let prev = countAndSay(n - 1);
  let result = '';
  let count = 1;
  for (let i = 1; i < prev.length; i++) {
    if (prev[i] === prev[i - 1]) {
      count++;
    } else {
      result += count + prev[i - 1];
      count = 1;
    }
  }
  result += count + prev[prev.length - 1];
  return result;
}
console.log(countAndSay(4));

medium

A. "1211"

B. "111221"

C. "21"

D. "312211"

Solution

1. Step 1: Trace recursive calls for n=4

   countAndSay(1) = '1'
   countAndSay(2) = '11' (one 1)
   countAndSay(3) = '21' (two 1s)
   countAndSay(4) = '1211' (one 2, one 1)

2. Step 2: Confirm output matches '1211'

   The code builds the string by counting consecutive digits correctly, so output is '1211'.

3. Final Answer:

   "1211" -> Option A

4. Quick Check:

   4th term = "1211" [OK]

Hint: Remember sequence: 1, 11, 21, 1211, ... [OK]

Common Mistakes:

• Mixing up counts and digits order
• Off-by-one errors in loops
• Confusing recursive base case

4. Identify the bug in this Count and Say function and select the fix:

function countAndSay(n) {
  if (n === 1) return '1';
  let prev = countAndSay(n - 1);
  let result = '';
  let count = 1;
  for (let i = 1; i < prev.length; i++) {
    if (prev[i] === prev[i - 1]) {
      count++;
    } else {
      result += count + prev[i - 1];
      count = 1;
    }
  }
  return result;
}

medium

A. Add result += count + prev[prev.length - 1]; after loop

B. Initialize count = 0 instead of 1

C. Change loop to for (let i = 1; i < prev.length; i++)

D. Return prev instead of result

Solution

1. Step 1: Analyze loop and indexing

   The loop starts from i=1 and compares prev[i] with prev[i-1]. It increments count for consecutive same digits but only appends when a change is detected (else block). The last group is never appended because the loop ends without processing the trailing group.

2. Step 2: Identify missing last group addition

   The code lacks appending the final count and digit after the loop. Adding result += count + prev[prev.length - 1]; after loop ensures the last group is included.

3. Final Answer:

   Add result += count + prev[prev.length - 1]; after loop -> Option A

4. Quick Check:

   Include last group after loop [OK]

Hint: Always add last counted group after loop [OK]

Common Mistakes:

• Forgetting to add last group after loop
• Changing loop start incorrectly
• Returning wrong variable

5. Given the Count and Say sequence, what is the 6th term?
Use the sequence starting with '1' as the 1st term.

hard

A. "1113213211"

B. "13112221"

C. "312211"

D. "211213"

Solution

1. Step 1: List terms up to 6th

   1st: '1'
   2nd: '11' (one 1)
   3rd: '21' (two 1s)
   4th: '1211' (one 2, one 1)
   5th: '111221' (one 1, one 2, two 1s)
   6th: '312211' (three 1s, two 2s, one 1)

2. Step 2: Confirm 6th term matches option

   "312211" matches '312211' exactly.

3. Final Answer:

   "312211" -> Option C

4. Quick Check:

   6th term = "312211" [OK]

Hint: Build terms stepwise to find nth term [OK]

Common Mistakes:

• Skipping terms or miscounting digits
• Mixing digit counts and digits
• Choosing longer incorrect strings