Count and Say Problem in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Typescript

let 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

Starting with "0" instead of "1".

Using multiple digits as the first term.

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

2fill in blank

medium

Complete the code to iterate through the current term characters.

DSA Typescript

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

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

Ai+=0

Bi+=2

Ci--

Di++

Attempts:

3 left

💡 Hint

Common Mistakes

Decrementing the index causing infinite loop.

Skipping characters by incrementing by 2.

We increment i by 1 to move through each character in the string one by one.

3fill in blank

hard

Fix the error in the condition to check if the next character is the same as the current.

DSA Typescript

while (j < current.length && current[j] [1] current[i]) {

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

A!=

B==

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' which breaks counting repeated characters.

Using greater or less than which is not meaningful for characters here.

We want to continue while the next character is equal to the current character to count repeats.

4fill in blank

hard

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

DSA Typescript

nextTerm += [1] + [2];

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

Acount.toString()

Bcount

Ccurrent[i]

D"count"

Attempts:

3 left

💡 Hint

Common Mistakes

Appending count as a number causing type errors.

Appending the string "count" instead of the variable.

We convert count to string and append the current character to form the next term.

5fill in blank

hard

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

DSA Typescript

for (let [1] = 1; [2] <= n; [3]++) {
  let nextTerm = "";
  // build nextTerm here
  result = nextTerm;
}

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

Ai

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the loop parts.

Using 'j' which is not declared for this loop.

The loop variable is 'i' starting at 1, running until i <= n, incrementing i by 1 each time.

Practice

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

easy

A. It describes each term by counting and saying the digits of the previous term.

B. It sorts the digits of the previous term in ascending order.

C. It adds the digits of the previous term to get the next term.

D. It reverses the digits of the previous term to form the next term.

Solution

1. Step 1: Understand the sequence definition

   The "Count and Say" sequence generates each term by describing the count of digits seen in the previous term.

2. Step 2: Compare options with definition

   Only It describes each term by counting and saying the digits of the previous term. correctly states this process; others describe unrelated operations.

3. Final Answer:

   It describes each term by counting and saying the digits of the previous term. -> Option A

4. Quick Check:

   Count and Say = Describe digits count [OK]

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

Common Mistakes:

• Thinking it sorts digits instead of counting
• Assuming it sums digits for next term
• Confusing with reversing digits

2. Which of the following TypeScript function signatures correctly represents a function that takes a number n and returns the nth term of the Count and Say sequence as a string?

easy

A. function countAndSay(n: number): string

B. function countAndSay(n: string): number

C. function countAndSay(): number

D. function countAndSay(n: number): number[]

Solution

1. Step 1: Identify input and output types

   The function should take a number n (term index) and return a string representing that term.

2. Step 2: Match signature with requirements

   function countAndSay(n: number): string matches input as number and output as string; others mismatch types or parameters.

3. Final Answer:

   function countAndSay(n: number): string -> Option A

4. Quick Check:

   Input number, output string [OK]

Hint: Input is term number, output is sequence string [OK]

Common Mistakes:

• Using wrong input type (string instead of number)
• Returning number or array instead of string
• Missing input parameter

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

function countAndSay(n: number): string {
  if (n === 1) return "1";
  const 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;
}
console.log(countAndSay(4));

medium

A. "21"

B. "111221"

C. "312211"

D. "1211"

Solution

1. Step 1: Trace recursive calls and terms

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

2. Step 2: Confirm output for n=4

   The code returns "1211" for n=4, matching "1211".

3. Final Answer:

   "1211" -> Option D

4. Quick Check:

   Term 4 = "1211" [OK]

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

Common Mistakes:

• Confusing term 3 and 4 outputs
• Off-by-one errors in loop
• Miscounting repeated digits

4. Identify the bug in this TypeScript implementation of the Count and Say sequence:

function countAndSay(n: number): string {
  if (n === 1) return "1";
  const prev = countAndSay(n - 1);
  let result = "";
  let count = 1;
  for (let i = 0; i < prev.length; i++) {
    if (prev[i] === prev[i + 1]) {
      count++;
    } else {
      result += count + prev[i];
      count = 1;
    }
  }
  return result;
}

medium

A. The count variable is not reset correctly inside the loop.

B. The loop should start from i = 1, not 0.

C. Accessing prev[i + 1] causes out-of-bounds error on last iteration.

D. The base case should return "0" instead of "1".

Solution

1. Step 1: Analyze loop boundary and indexing

   The loop runs from i=0 to prev.length-1, but accesses prev[i+1], which is undefined at last index.

2. Step 2: Identify error cause

   Accessing prev[i+1] at last iteration causes out-of-bounds access, leading to incorrect behavior or runtime error.

3. Final Answer:

   Accessing prev[i + 1] causes out-of-bounds error on last iteration. -> Option C

4. Quick Check:

   Check array bounds when accessing i+1 [OK]

Hint: Check array access at last index carefully [OK]

Common Mistakes:

• Ignoring last element boundary
• Starting loop at wrong index
• Incorrect base case value

5. Given the Count and Say sequence, what is the 6th term generated by the following TypeScript function?

function countAndSay(n: number): string {
  if (n === 1) return "1";
  const 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;
}
console.log(countAndSay(6));

hard

A. "13112221"

B. "312211"

C. "1113213211"

D. "21121314"

Solution

1. Step 1: List terms up to 6

   Term 1: "1"; Term 2: "11"; Term 3: "21"; Term 4: "1211"; Term 5: "111221"; Term 6: "312211".

2. Step 2: Confirm output matches term 6

   The function returns "312211" for n=6, matching "312211".

3. Final Answer:

   "312211" -> Option B

4. Quick Check:

   Term 6 = "312211" [OK]

Hint: Memorize first 6 terms or trace stepwise [OK]

Common Mistakes:

• Mixing up term 5 and 6
• Miscounting repeated digits
• Skipping recursive calls