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

Recall & Review

beginner

What is the main idea behind finding the first non-repeating character using a hash?

Use a hash (dictionary) to count how many times each character appears, then find the first character with count 1.

beginner

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

First pass counts characters, second pass finds the first character with count 1.

beginner

What data structure is commonly used to store character counts for this problem?

A dictionary or hash map where keys are characters and values are counts.

beginner

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

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

intermediate

How does using a hash improve the efficiency compared to checking each character against all others?

Hash allows counting in O(n) time instead of O(n²), making the solution faster.

What is the time complexity of finding the first non-repeating character using a hash map?

A O(n)

B O(n²)

C O(log n)

D O(1)

Counting characters and then scanning once both take O(n), so total is O(n).

Which data structure is best to store character counts for this problem?

A Stack

B Queue

C Linked List

D Dictionary (Hash Map)

Dictionary allows fast lookup and update of counts by character.

If the input string is "aabbcc", what should the function return?

A -1

B a

C 0

D c

All characters repeat, so return -1 indicating no non-repeating character.

Why do we need to scan the string a second time after counting characters?

A To sort the characters

B To print the string

C To find the first character with count 1

D To reverse the string

The second scan finds the first character that appears only once.

What happens if you use a list instead of a dictionary for counting characters?

A It works faster

B It may be slower or complicated for large character sets

C It uses less memory

D It automatically sorts characters

Lists are less efficient for direct character counting unless characters are limited and mapped.

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

Think about counting first, then searching.

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

Focus on time complexity differences.

Practice

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

easy

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

B. To sort the characters alphabetically

C. To reverse the string quickly

D. To store the string length

Solution

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

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

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

   By checking the counts, 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 in the string -> Option A

4. Quick Check:

   Hash = frequency map [OK]

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

Common Mistakes:

• Trying to sort characters instead of counting
• Using hash to store indexes only
• Ignoring character frequency

2. Which of the following C# code snippets correctly initializes a dictionary to count character frequencies for a string input?

easy

A. var freq = new Dictionary(); foreach (char c in input) { if (freq.ContainsKey(c)) freq[c]++; else freq[c] = 1; }

B. var freq = new Dictionary(); foreach (char c in input) { freq[c] = 1; }

C. var freq = new List<int>(); foreach (char c in input) { freq.Add(c); }

D. var freq = new Dictionary(); foreach (char c in input) { freq[c] = c.ToString(); }

Solution

1. Step 1: Check dictionary key and value types

   We need a dictionary with char keys and int values to count frequencies.

2. Step 2: Verify frequency counting logic

   var freq = new Dictionary(); foreach (char c in input) { if (freq.ContainsKey(c)) freq[c]++; else freq[c] = 1; } correctly checks if the character exists and increments count or sets it to 1.

3. Final Answer:

   var freq = new Dictionary(); foreach (char c in input) { if (freq.ContainsKey(c)) freq[c]++; else freq[c] = 1; } -> Option A

4. Quick Check:

   Correct dictionary type and counting logic [OK]

Hint: Use Dictionary and increment counts properly [OK]

Common Mistakes:

• Using wrong dictionary types
• Not incrementing counts correctly
• Using List instead of Dictionary

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

string input = "swiss";
var freq = new Dictionary();
foreach (char c in input) {
  if (freq.ContainsKey(c)) freq[c]++;
  else freq[c] = 1;
}
char result = '\0';
foreach (char c in input) {
  if (freq[c] == 1) {
    result = c;
    break;
  }
}
Console.WriteLine(result == '\0' ? "None" : result);

medium

A. i

B. s

C. w

D. None

Solution

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

   s appears 3 times, w appears 1 time, i appears 1 time.

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

   Order is s, w, i, s, s. The first with count 1 is 'w'.

3. Final Answer:

   w -> Option C

4. Quick Check:

   First unique char = w [OK]

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

Common Mistakes:

• Choosing 'i' because it is unique but not first
• Choosing 's' which repeats
• Printing None incorrectly

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

string input = "hello";
var freq = new Dictionary();
foreach (char c in input) {
  freq[c] = freq[c] + 1;
}
char result = '\0';
foreach (char c in input) {
  if (freq[c] == 1) {
    result = c;
    break;
  }
}
Console.WriteLine(result == '\0' ? "None" : result);

medium

A. The result variable is not initialized

B. freq[c] is used before initialization causing KeyNotFoundException

C. The loop does not iterate over the string

D. The output statement is missing

Solution

1. Step 1: Analyze frequency counting loop

   The code tries to increment freq[c] without checking if c exists in dictionary, causing error.

2. Step 2: Identify correct fix

   Must check if freq contains c before incrementing or initialize freq[c] to 1 if missing.

3. Final Answer:

   freq[c] is used before initialization causing KeyNotFoundException -> Option B

4. Quick Check:

   Accessing uninitialized key causes error [OK]

Hint: Always check key exists before incrementing dictionary value [OK]

Common Mistakes:

• Assuming dictionary auto-initializes keys
• Ignoring exception details
• Missing initialization logic

5. Given the string input = "aabbccddeeffg", which character will be the first non-repeating character using a hash-based approach? If none exists, return 'None'.

hard

A. f

B. a

C. None

D. g

Solution

1. Step 1: Count frequencies of each character

   Characters a,b,c,d,e,f appear twice each; g appears once.

2. Step 2: Find first character with frequency 1 in string order

   String order is a,a,b,b,c,c,d,d,e,e,f,f,g. The first with count 1 is 'g'.

3. Final Answer:

   g -> Option D

4. Quick Check:

   Only 'g' is unique and first non-repeating [OK]

Hint: Check counts, then scan string order for first unique char [OK]

Common Mistakes:

• Choosing 'None' incorrectly
• Picking repeated characters
• Ignoring last unique character