First Non Repeating Character Using Hash in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty hash map for character counts.

DSA Typescript

const charCount: Record<string, number> = [1];

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

Anull

B{}

C[]

Dnew Map()

Attempts:

3 left

💡 Hint

Common Mistakes

Using [] creates an array, not a hash map.

Using null causes runtime errors.

new Map() is a different data structure, not an object.

We use an empty object {} to store character counts as keys and their frequencies as values.

2fill in blank

medium

Complete the code to increment the count of a character in the hash map.

DSA Typescript

charCount[char] = (charCount[char] || 0) [1] 1;

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

A-

B/

C+

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction decreases the count incorrectly.

Multiplication or division does not increment the count.

We add 1 to the existing count or 0 if the character is not yet in the map.

3fill in blank

hard

Fix the error in the loop that finds the first non-repeating character.

DSA Typescript

for (const [1] of s) {
  if (charCount[char] === 1) {
    return char;
  }
}

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

Achar

Bi

Cindex

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name different from the one inside the loop causes reference errors.

The loop variable should be 'char' to match the usage inside the loop.

4fill in blank

hard

Fill both blanks to complete the function that returns the first non-repeating character or '_' if none found.

DSA Typescript

function firstNonRepeatingChar(s: string): string {
  const charCount: Record<string, number> = {};
  for (const [1] of s) {
    charCount[[2]] = (charCount[[2]] || 0) + 1;
  }
  for (const char of s) {
    if (charCount[char] === 1) {
      return char;
    }
  }
  return '_';
}

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

Achar

Bcount

Cindex

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causes undefined key errors.

The loop variable and the key used in the hash map must both be 'char' to count characters correctly.

5fill in blank

hard

Fill all three blanks to create a dictionary of characters that appear exactly once.

DSA Typescript

const result: Record<string, number> = {};
for (const [3] of s) {
  if (charCount[[3]] === 1) {
    result[[1]] = [2];
  }
}

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

Achar

BcharCount[char]

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names causes errors.

Not filtering by count === 1 includes repeating characters.

We use 'char' as the key and 'charCount[char]' as the value for characters that appear once.

Practice

1. What is the main purpose of using a hash (like a dictionary or map) to find the first non-repeating character in a string?

easy

A. To count the frequency of each character efficiently

B. To sort the characters alphabetically

C. To reverse the string quickly

D. To remove duplicate characters from the string

Solution

1. Step 1: Understand the role of a hash in frequency counting

   A hash (dictionary/map) stores each character as a key and counts how many times it appears in the string.

2. Step 2: Use frequency counts to find the first non-repeating character

   By checking the counts in order, we can find the first character with count 1, which is the first non-repeating character.

3. Final Answer:

   To count the frequency of each character efficiently -> Option A

4. Quick Check:

   Hash counts characters = To count the frequency of each character efficiently [OK]

Hint: Use a hash to count characters, then find count 1 [OK]

Common Mistakes:

• Thinking hash sorts characters
• Trying to reverse string instead
• Assuming hash removes duplicates

2. Which of the following TypeScript code snippets correctly initializes a hash map to store character counts?

easy

A. const charCount = new Set<string>();

B. const charCount = new Array();

C. const charCount = new Map();

D. const charCount = new Object<number>();

Solution

1. Step 1: Identify correct TypeScript syntax for a map

   TypeScript uses Map to create a hash map. Here, keys are strings and values are numbers.

2. Step 2: Check each option for correctness

   const charCount = new Map(); uses Map(), which is correct. Options A, B, and D use incorrect types or structures.

3. Final Answer:

   const charCount = new Map<string, number>(); -> Option C

4. Quick Check:

   Map<string, number> is correct syntax [OK]

Hint: Use Map() to store char counts [OK]

Common Mistakes:

• Using Array instead of Map
• Using Set which stores only keys
• Incorrect Object initialization

3. What will be the output of the following TypeScript code?

function firstNonRepeatingChar(s: string): string {
  const count = new Map();
  for (const ch of s) {
    count.set(ch, (count.get(ch) ?? 0) + 1);
  }
  for (const ch of s) {
    if (count.get(ch) === 1) return ch;
  }
  return '_';
}
console.log(firstNonRepeatingChar('swiss'));

medium

A. '_'

B. 's'

C. 'i'

D. 'w'

Solution

1. Step 1: Count character frequencies in 'swiss'

   Characters: s(3), w(1), i(1). The map stores these counts.

2. Step 2: Find first character with count 1 in order

   Order: 's' (count 3), 'w' (count 1), 'i' (count 1). The first with count 1 is 'w'.

3. Final Answer:

   'w' -> Option D

4. Quick Check:

   First unique char in 'swiss' = 'w' [OK]

Hint: Check counts in order, first with 1 is answer [OK]

Common Mistakes:

• Choosing first character without checking count
• Confusing 'i' and 'w'
• Returning '_' when unique char exists

4. Identify the bug in this TypeScript function to find the first non-repeating character:

function firstNonRepeatingChar(s: string): string {
  const count = new Map();
  for (let i = 0; i <= s.length; i++) {
    const ch = s[i];
    count.set(ch, (count.get(ch) ?? 0) + 1);
  }
  for (const ch of s) {
    if (count.get(ch) === 1) return ch;
  }
  return '_';
}

medium

A. Map should be initialized with Object instead of Map

B. Loop condition should be i < s.length, not i <= s.length

C. Return '_' should be inside the first loop

D. Use 'for...in' instead of 'for...of' for string iteration

Solution

1. Step 1: Check the for loop boundary

   The loop runs from i = 0 to i <= s.length, which accesses s[s.length], an undefined index causing error.

2. Step 2: Correct loop condition

   Change condition to i < s.length to avoid out-of-bounds access and correctly count characters.

3. Final Answer:

   Loop condition should be i < s.length, not i <= s.length -> Option B

4. Quick Check:

   Loop boundary off by one error [OK]

Hint: Use i < length, not i <= length in loops [OK]

Common Mistakes:

• Off-by-one error in loop condition
• Misusing Map initialization
• Wrong loop type for string iteration

5. Given the string 'aabbccddeeffg', which modification to the hash-based approach will correctly return the first non-repeating character when some characters appear multiple times and some only once?

hard

A. After counting, iterate the string and return the first character with count 1

B. Sort the string and return the first character

C. Return the character with the highest count from the hash

D. Return the last character in the string regardless of counts

Solution

1. Step 1: Count frequencies of all characters

   Use a hash map to count how many times each character appears in the string.

2. Step 2: Iterate string in original order to find first count 1

   Check each character in order; the first with count 1 is the first non-repeating character.

3. Final Answer:

   After counting, iterate the string and return the first character with count 1 -> Option A

4. Quick Check:

   Count then check order for first unique char [OK]

Hint: Count all, then find first with count 1 in order [OK]

Common Mistakes:

• Sorting string loses original order
• Returning highest count instead of unique
• Ignoring character order in result