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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of First Non Repeating Character Index

What is the output of the following JavaScript code that finds the index of the first non-repeating character in a string?

DSA Javascript

function firstNonRepeatingCharIndex(s) {
  const charCount = {};
  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(firstNonRepeatingCharIndex("swiss"));

A 3

B 2

C 1

D -1

💡 Hint

Check the count of each character and find the first with count 1.

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

❓ Predict Output

intermediate

Output of First Non Repeating Character Index with all repeating

What is the output of the following JavaScript code when all characters repeat?

DSA Javascript

function firstNonRepeatingCharIndex(s) {
  const charCount = {};
  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(firstNonRepeatingCharIndex("aabbcc"));

A 6

B 0

C 1

D -1

💡 Hint

If no character appears once, the function returns -1.

All characters in "aabbcc" appear twice, so no non-repeating character exists. The function returns -1.

🔧 Debug

advanced

Identify the error in this First Non Repeating Character code

What error does the following code produce when run?

DSA Javascript

function firstNonRepeatingCharIndex(s) {
  const charCount = new Map();
  for (const ch of s) {
    if (charCount.has(ch)) {
      charCount.set(ch, charCount.get(ch) + 1);
    } else {
      charCount.set(ch, 1);
    }
  }
  for (let i = 0; i < s.length; i++) {
    if (charCount.get(s[i]) === 1) return i;
  }
  return -1;
}

console.log(firstNonRepeatingCharIndex("hello"));

A No error, outputs -1

B TypeError: charCount.set is not a function

C TypeError: charCount.get is not a function

D TypeError: charCount.has is not a function

💡 Hint

Check how Map methods are used and how properties are assigned.

The code mixes Map methods with object property assignment. `charCount[ch] = 1` sets an own property on the Map instance, not a Map entry. `Map.has(ch)` and `Map.get(ch)` only check Map entries, which are never set, so `get(s[i])` returns undefined !== 1 for all, returns -1. No runtime error.

❓ Predict Output

advanced

Output of First Non Repeating Character with Unicode characters

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

DSA Javascript

function firstNonRepeatingCharIndex(s) {
  const charCount = {};
  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(firstNonRepeatingCharIndex("aáa"));

A -1

B 1

C 2

D 0

💡 Hint

Unicode characters are treated as distinct keys in the hash.

The string "aáa" has 'a' twice and 'á' once at index 1. So the first non-repeating character is at index 1.

🚀 Application

expert

How many unique characters are counted in this string?

Using the hash counting method, how many unique characters does the string "abcabcdeff" have?

DSA Javascript

function countUniqueChars(s) {
  const charCount = {};
  for (const ch of s) {
    charCount[ch] = (charCount[ch] || 0) + 1;
  }
  return Object.keys(charCount).length;
}

console.log(countUniqueChars("abcabcdeff"));

A 7

B 8

C 6

D 9

💡 Hint

Count distinct characters in the string.

The string "abcabcdeff" has unique characters: a, b, c, d, e, f (6 unique).

Practice

1. What is the main purpose of using a hash (object) in finding the first non repeating character in a string?

easy

A. To reverse the string

B. To sort the characters alphabetically

C. To remove duplicate characters from the string

D. To count the frequency of each character in the string

Solution

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

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

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

   By checking the hash, we find the first character with count 1, which is the first non repeating character.

3. Final Answer:

   To count the frequency of each character in the string -> Option D

4. Quick Check:

   Hash counts characters = Frequency count [OK]

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

Common Mistakes:

• Trying to sort characters instead of counting
• Ignoring character frequency
• Using hash to reverse string

2. Which of the following is the correct syntax to initialize an empty hash (object) in JavaScript for counting characters?

easy

A. let hash = [];

B. let hash = {};

C. let hash = ();

D. let hash = '';

Solution

1. Step 1: Identify JavaScript object syntax

   In JavaScript, curly braces {} create an empty object, which is used as a hash for key-value pairs.

2. Step 2: Confirm correct initialization

   Using let hash = {}; correctly initializes an empty object for counting characters.

3. Final Answer:

   let hash = {}; -> Option B

4. Quick Check:

   Empty object = {} [OK]

Hint: Use {} to create empty object for hash in JavaScript [OK]

Common Mistakes:

• Using [] which creates an array, not an object
• Using () which is invalid syntax
• Using empty string '' which is not an object

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

function firstNonRepeatingChar(str) {
  const hash = {};
  for (const ch of str) {
    hash[ch] = (hash[ch] || 0) + 1;
  }
  for (const ch of str) {
    if (hash[ch] === 1) return ch;
  }
  return '_';
}

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

medium

A. 'w'

B. 's'

C. 'i'

D. '_'

Solution

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

   Characters: s(3), w(1), i(1). Hash = {s:3, w:1, i:1}

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

   Order: s, w, i, s, s. First with count 1 is 'w'.

3. Final Answer:

   'w' -> Option A

4. Quick Check:

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

Hint: Check counts, then scan string for first count 1 char [OK]

Common Mistakes:

• Returning 's' which repeats
• Returning 'i' which is second unique
• Returning '_' which means no unique char

4. Identify the error in the following code snippet that tries to find the first non repeating character:

function firstNonRepeatingChar(str) {
  const hash = {};
  for (let i = 0; i <= str.length; i++) {
    const ch = str[i];
    hash[ch] = (hash[ch] || 0) + 1;
  }
  for (let i = 0; i < str.length; i++) {
    if (hash[str[i]] === 1) return str[i];
  }
  return '_';
}

medium

A. The hash is not initialized correctly

B. The second loop should use <= instead of <

C. The first loop uses <= instead of < causing undefined access

D. The return '_' is unreachable code

Solution

1. Step 1: Check loop conditions for index errors

   The first loop uses i <= str.length, which accesses str[str.length] (undefined), causing an error.

2. Step 2: Correct loop condition to i < str.length

   Changing to i < str.length prevents out-of-bounds access and correctly counts characters.

3. Final Answer:

   The first loop uses <= instead of < causing undefined access -> Option C

4. Quick Check:

   Loop index must be less than length [OK]

Hint: Use < not <= for string index loops to avoid undefined [OK]

Common Mistakes:

• Using <= causes out-of-range errors
• Ignoring loop boundary conditions
• Misunderstanding hash initialization

5. Given the string 'aabbccddeeffg', which modification to the code below will correctly return the first non repeating character?

function firstNonRepeatingChar(str) {
  const hash = {};
  for (const ch of str) {
    hash[ch] = (hash[ch] || 0) + 1;
  }
  for (const ch of str) {
    if (hash[ch] === 1) return ch;
  }
  return '_';
}

Options:
A) Change return '_' to return null
B) Add a check to ignore spaces in the string
C) Modify the function to return '_' if no unique character exists
D) Change the hash to store boolean instead of counts

hard

A. Change return '_' to return null

B. Add a check to ignore spaces in the string

C. Modify the function to return '_' if no unique character exists

D. Change the hash to store boolean instead of counts

Solution

1. Step 1: Analyze the string and current function behavior

   String 'aabbccddeeffg' has all characters repeated except 'g' which appears once.

2. Step 2: Confirm function returns '_' if no unique char

   The function already returns '_' if no unique character is found, which is correct behavior.

3. Step 3: Understand why changing return '_' to return null is better

   Changing return '_' to return null clearly indicates no unique character found, which is a better practice than returning a character that might be confused as valid.

4. Final Answer:

   Change return '_' to return null -> Option A

5. Quick Check:

   Return null when no unique char found is clearer [OK]

Hint: Return null to indicate no unique character found [OK]

Common Mistakes:

• Returning '_' which can be confused as a valid character
• Ignoring spaces when not needed
• Using boolean instead of counts loses frequency info