First Non Repeating Character Using Hash in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using a hash (object) in finding the first non-repeating character?

A hash stores the count of each character in the string, allowing quick lookup to find which character appears only once.

beginner

How do you identify the first non-repeating character after counting frequencies?

By scanning the string from start to end and checking the hash for characters with count 1, the first such character is the answer.

intermediate

Why is using a hash efficient for this problem compared to nested loops?

Using a hash reduces time complexity from O(n²) to O(n) by avoiding repeated counting for each character.

beginner

What would be the output for the input string 'swiss' when finding the first non-repeating character?

The first non-repeating character is 'w' because 's' repeats multiple times and 'w' appears only once first.

beginner

What data structure is commonly used as a hash in JavaScript for this problem?

A plain JavaScript object or Map can be used as a hash to store character counts.

What is the first step in finding the first non-repeating character using a hash?

A Count the frequency of each character

B Sort the string

C Reverse the string

D Remove all repeating characters

Counting the frequency of each character helps identify which characters appear only once.

After counting characters, how do you find the first non-repeating character?

A Pick any character with count more than 1

B Pick the character with the highest count

C Pick the last character in the string

D Check characters in order and find the first with count 1

You scan the string from start to end and pick the first character with count 1.

What is the time complexity of the hash-based approach to find the first non-repeating character?

A O(n)

B O(n²)

C O(log n)

D O(1)

The approach takes linear time because it scans the string twice: once to count and once to find.

Which JavaScript data structure is best suited for counting characters in this problem?

A Array

B Object or Map

C Set

D String

Objects or Maps store key-value pairs, perfect for character counts.

What will be the output for input 'aabbcc' when finding the first non-repeating character?

A c

B a

C No non-repeating character

D b

All characters repeat, so there is no non-repeating character.

Explain step-by-step how to find the first non-repeating character in a string using a hash.

Think about counting first, then scanning.

Why is using a hash better than nested loops for this problem?

Consider how many times you count characters.

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