First Non Repeating Character Using Hash in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a dictionary that stores character counts.

DSA C#

var charCount = new Dictionary<char, [1]>();

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Achar

Bbool

Cint

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using string or char as the value type instead of int.

Forgetting to specify the value type in the dictionary.

The dictionary stores characters as keys and their counts as integer values.

2fill in blank

medium

Complete the code to check if the dictionary contains the character key.

DSA C#

if (!charCount.[1](c)) { charCount[c] = 1; } else { charCount[c]++; }

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AContainsKey

BContainsValue

CAdd

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add instead of ContainsKey, which causes errors.

Using ContainsValue which checks values, not keys.

ContainsKey checks if the dictionary already has the character as a key.

3fill in blank

hard

Fix the error in the loop that finds the first non-repeating character.

DSA C#

foreach (var c in s) { if (charCount[c] [1] 1) { return c; } }

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A=

B==

C>=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which assigns instead of compares.

Using '!=' which finds characters that repeat.

We check if the count equals 1 to find the first non-repeating character.

4fill in blank

hard

Fill both blanks to complete the method signature and return statement.

DSA C#

public static char [1](string [2]) { /* method body */ return '\0'; }

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AFirstNonRepeatingChar

Bs

Cinput

DFindChar

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent parameter names.

Using vague method names.

The method is named FirstNonRepeatingChar and takes a string parameter named s.

5fill in blank

hard

Fill all three blanks to complete the dictionary update and loop.

DSA C#

foreach (char [1] in [2]) { if (!charCount.ContainsKey([1])) { charCount[[1]] = 1; } else { charCount[[1]][3]; } }

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Ac

Bs

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names in the loop.

Using decrement operator instead of increment.

We loop over each character c in string s and increment its count with ++.

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