First Non Repeating Character Using Hash in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Find the first non-repeating character index

What is the output of this TypeScript code that finds the index of the first non-repeating character in the string?

DSA Typescript

function firstNonRepeatingChar(s: string): number {
  const charCount: {[key: string]: number} = {};
  for (const ch of s) {
    charCount[ch] = (charCount[ch] || 0) + 1;
  }
  for (let i = 0; i < s.length; i++) {
    if (charCount[s[i]] === 1) {
      return i;
    }
  }
  return -1;
}

console.log(firstNonRepeatingChar("swiss"));

A 1

B 0

C 2

D -1

💡 Hint

Count how many times each character appears, then find the first with count 1.

The string "swiss" has characters: s(3 times), w(1 time at index 1), i(1 time at index 2). The first non-repeating character is 'w' at index 1.

❓ Predict Output

intermediate

Output of first non-repeating character function

What does this code print when given the input "aabbccdde"?

DSA Typescript

function firstNonRepeatingChar(s: string): number {
  const counts: {[key: string]: number} = {};
  for (const ch of s) {
    counts[ch] = (counts[ch] || 0) + 1;
  }
  for (let i = 0; i < s.length; i++) {
    if (counts[s[i]] === 1) {
      return i;
    }
  }
  return -1;
}

console.log(firstNonRepeatingChar("aabbccdde"));

A 8

B 7

C -1

D 0

💡 Hint

Look for the first character that appears only once.

Characters and counts: a(2), b(2), c(2), d(2), e(1). 'e' is the only non-repeating character at index 8.

🔧 Debug

advanced

Identify the error in this first non-repeating character code

What error will this TypeScript code produce when run?

DSA Typescript

function firstNonRepeatingChar(s: string): number {
  const counts = new Map<string, number>();
  for (const ch of s) {
    if (counts.has(ch)) {
      counts.set(ch, counts.get(ch)! + 1);
    } else {
      counts.set(ch, 1);
    }
  }
  for (let i = 0; i < s.length; i++) {
    if (counts.get(s[i]) === 1) {
      return i;
    }
  }
  return -1;
}

console.log(firstNonRepeatingChar("abcabcde"));

A TypeError: Cannot read property 'get' of undefined

B RangeError: Maximum call stack size exceeded

C No error, outputs 6

D undefined is not a function

💡 Hint

Check the loop condition for accessing string indices.

There is no runtime error because the first non-repeating character 'd' is found at index 6 before i reaches s.length. However, the loop condition i <= s.length is incorrect and could access s[s.length] (undefined) in cases with no early return.

❓ Predict Output

advanced

Output of first non-repeating character with all repeating chars

What is the output of this code when all characters repeat?

DSA Typescript

function firstNonRepeatingChar(s: string): number {
  const counts: {[key: string]: number} = {};
  for (const ch of s) {
    counts[ch] = (counts[ch] || 0) + 1;
  }
  for (let i = 0; i < s.length; i++) {
    if (counts[s[i]] === 1) {
      return i;
    }
  }
  return -1;
}

console.log(firstNonRepeatingChar("aabbcc"));

A 0

B 6

C 5

D -1

💡 Hint

If no character appears once, what should the function return?

All characters repeat, so no non-repeating character exists. The function returns -1.

❓ Predict Output

expert

Output of first non-repeating character with Unicode characters

What is the output of this code when the input string contains Unicode characters?

DSA Typescript

function firstNonRepeatingChar(s: string): number {
  const counts: {[key: string]: number} = {};
  for (const ch of s) {
    counts[ch] = (counts[ch] || 0) + 1;
  }
  for (let i = 0; i < s.length; i++) {
    if (counts[s[i]] === 1) {
      return i;
    }
  }
  return -1;
}

console.log(firstNonRepeatingChar("😀😃😄😀😃"));

A 2

B -1

C 3

D 0

💡 Hint

Consider how Unicode characters are counted and indexed in strings.

JavaScript strings use UTF-16 encoding. The for...of loop iterates over code points (full emojis as keys in counts), but s[i] accesses 16-bit code units (surrogate halves). The keys do not match, so counts[s[i]] is always undefined, and the function returns -1.

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