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

Recall & Review

beginner

What is the main idea behind using a hash map to find the first non-repeating character in a string?

Use a hash map to count how many times each character appears, then find the first character with a count of one.

beginner

Why do we need two passes over the string when using a hash map to find the first non-repeating character?

First pass counts characters, second pass finds the first character with count one.

beginner

In TypeScript, which data structure is best to store character counts for this problem?

A Map or a plain object can be used to store counts of each character.

beginner

What should the function return if all characters in the string repeat?

Return -1 or a special value indicating no non-repeating character found.

intermediate

How does the hash map approach compare to checking each character against all others?

Hash map approach is faster (linear time) because it avoids repeated scanning of the string.

What does the hash map store in the 'First Non Repeating Character Using Hash' approach?

A Count of each character in the string

B Index of each character

C Characters sorted alphabetically

D Length of the string

The hash map stores how many times each character appears.

How many times do we scan the string in the hash map method?

A Twice

B Three times

C Once

D Four times

First to count characters, second to find the first non-repeating.

What is the time complexity of the hash map approach for this problem?

A O(n^2)

B O(n log n)

C O(n)

D O(1)

Counting and scanning are both linear in string length.

If the string is 'aabbcc', what should the function return?

A 0

B -1

C a

D c

All characters repeat, so return -1.

Which TypeScript type is suitable for the hash map in this problem?

A Array<string>

B number

C Set<string>

D Map<string, number>

Map stores character counts efficiently.

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

Think about counting first, then searching.

Describe why using a hash map is better than nested loops for this problem.

Focus on time complexity and efficiency.

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