First Non Repeating Character Using Hash in DSA Typescript - Time & Space Complexity

Time Complexity: First Non Repeating Character Using Hash

O(n)

Understanding Time Complexity

We want to know how the time to find the first non-repeating character changes as the input string gets longer.

How does the number of steps grow when the string size increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function firstNonRepeatingChar(s: string): string | null {
  const charCount: Record = {};
  for (const char of s) {
    charCount[char] = (charCount[char] || 0) + 1;
  }
  for (const char of s) {
    if (charCount[char] === 1) return char;
  }
  return null;
}
    

This code counts how many times each character appears, then finds the first character that appears only once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Two loops over the string characters.
• How many times: Each loop runs once over all characters, so twice over the input length.

How Execution Grows With Input

As the string gets longer, the number of steps grows roughly twice as fast because we scan the string two times.

Input Size (n)	Approx. Operations
10	About 20 steps
100	About 200 steps
1000	About 2000 steps

Pattern observation: The steps grow linearly with input size, doubling because of two passes.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the first non-repeating character grows directly in proportion to the string length.

Common Mistake

[X] Wrong: "Because there are two loops, the time complexity is O(n²)."

[OK] Correct: The loops run one after another, not nested. So the total steps add up, not multiply.

Interview Connect

Understanding this helps you explain how to efficiently find unique elements using counting, a common skill in coding interviews.

Self-Check

"What if we used a nested loop to check each character against all others? How would the time complexity change?"

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