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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a hash map for character counts.

DSA C++

std::unordered_map<char, int> [1];

Drag options to blanks, or click blank then click option'

AcharCount

Bfreq

CcountMap

DhashMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using an invalid variable name.

Forgetting to specify the template parameters.

We declare an unordered_map named charCount to store character frequencies.

2fill in blank

medium

Complete the code to increment the count of each character in the string.

DSA C++

for (char c : s) {
    [1][c]++;
}

Drag options to blanks, or click blank then click option'

AcountMap

BcharCount

Cfreq

DhashMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than declared.

Forgetting to increment the count.

We use charCount[c]++ to increase the count of character c in the map.

3fill in blank

hard

Fix the error in the code to find the first non-repeating character.

DSA C++

for (char c : s) {
    if ([1][c] == 1) {
        return c;
    }
}

Drag options to blanks, or click blank then click option'

AcountMap

BhashMap

Cfreq

DcharCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong variable name causing compilation error.

Checking for count != 1 instead of == 1.

We check charCount[c] to find characters that appear exactly once.

4fill in blank

hard

Fill the blanks to complete the function that returns the first non-repeating character or '_' if none found.

DSA C++

char firstNonRepeatingChar(const std::string& s) {
    std::unordered_map<char, int> [1];
    for (char c : s) {
        [2][c]++;
    }
    for (char c : s) {
        if ([3][c] == 1) {
            return c;
        }
    }
    return '_';
}

Drag options to blanks, or click blank then click option'

AcharCount

BcountMap

Cfreq

DhashMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causing errors.

Forgetting to return '_' when no unique character is found.

The same variable charCount is used for declaration and accessing counts.

5fill in blank

hard

Fill all three blanks to create a map comprehension-like code that counts characters and finds the first unique one.

DSA C++

std::unordered_map<char, int> [1];
for (char [2] : s) {
    [1][[2]]++;
}

Drag options to blanks, or click blank then click option'

AcharCount

Bc

CcountMap

Dch

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using undeclared variables.

We declare charCount, iterate with variable c, and update charCount[c].

Practice

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

easy

A. To reverse the string efficiently

B. To sort the characters alphabetically

C. To store characters in a linked list

D. To store the frequency of each character for quick lookup

Solution

1. Step 1: Understand the role of hash map

   A hash map stores each character as a key and its frequency as the value, allowing quick frequency lookup.

2. Step 2: Why frequency is important

   Knowing frequency helps identify which character appears only once, which is the first non-repeating character.

3. Final Answer:

   To store the frequency of each character for quick lookup -> Option D

4. Quick Check:

   Hash map = frequency storage [OK]

Hint: Hash map counts characters fast for first unique [OK]

Common Mistakes:

• Thinking hash map sorts characters
• Using hash map to reverse string
• Confusing hash map with linked list

2. Which of the following C++ code snippets correctly declares a hash map to store character frequencies?

easy

A. std::unordered_map freq;

B. std::map freq;

C. std::unordered_map freq;

D. std::vector<int> freq(26);

Solution

1. Step 1: Identify correct hash map declaration

   We need a map with character keys and integer values for frequency, so std::unordered_map is correct.

2. Step 2: Why others are incorrect

   std::map freq; reverses key and value types; std::vector<int> freq(26); uses vector, not a hash map; std::unordered_map freq; uses string keys, not char.

3. Final Answer:

   std::unordered_map<char, int> freq; -> Option C

4. Quick Check:

   Correct map type = unordered_map<char, int> [OK]

Hint: Use unordered_map for char frequency [OK]

Common Mistakes:

• Using map with wrong key-value types
• Using vector instead of map
• Using string keys instead of char

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

std::string s = "swiss";
std::unordered_map freq;
for (char c : s) freq[c]++;
for (char c : s) {
  if (freq[c] == 1) {
    std::cout << c << std::endl;
    break;
  }
}

medium

A. s

B. w

C. i

D. No output

Solution

1. Step 1: Count character frequencies

   Characters: s(3), w(1), i(1). Frequencies: s=3, w=1, i=1.

2. Step 2: Find first character with frequency 1

   Iterate string: 's'(3), 'w'(1) -> first non-repeating is 'w'.

3. Final Answer:

   w -> Option B

4. Quick Check:

   First unique char = w [OK]

Hint: Check frequencies, print first with count 1 [OK]

Common Mistakes:

• Printing first character without checking frequency
• Confusing 'i' as first unique
• Ignoring frequency counts

4. Identify the error in the following code snippet that attempts to find the first non-repeating character:

std::string s = "hello";
std::unordered_map freq;
for (int i = 0; i <= s.size(); i++) {
  freq[s[i]]++;
}
for (int i = 0; i < s.size(); i++) {
  if (freq[s[i]] == 1) {
    std::cout << s[i] << std::endl;
    break;
  }
}

medium

A. Loop condition should be i < s.size(), not i <= s.size()

B. Frequency map should use string keys, not char

C. The break statement is missing

D. The output should be stored in a variable, not printed

Solution

1. Step 1: Check loop boundary in frequency count

   The loop uses i <= s.size(), which accesses s[s.size()] out of bounds causing undefined behavior.

2. Step 2: Correct loop boundary

   Change condition to i < s.size() to avoid out-of-range access.

3. Final Answer:

   Loop condition should be i < s.size(), not i <= s.size() -> Option A

4. Quick Check:

   Loop boundary off by one error [OK]

Hint: Use i < size(), not <= size() in loops [OK]

Common Mistakes:

• Using <= instead of < in loops
• Confusing char and string keys
• Thinking break is missing

5. Given the string "aabbccdde", which modification to the standard hash map approach will correctly find the first non-repeating character when some characters appear multiple times and others only once at the end?

hard

A. Store character frequencies and track insertion order using a queue

B. Sort the string before counting frequencies

C. Use a vector instead of a hash map for frequency

D. Only count characters until the first repeating character

Solution

1. Step 1: Understand problem with standard hash map

   Hash map stores frequencies but does not keep order, so first unique character position is lost.

2. Step 2: Use queue to track order

   By storing characters in a queue as they appear, and checking frequency, we can find the first non-repeating character in order.

3. Final Answer:

   Store character frequencies and track insertion order using a queue -> Option A

4. Quick Check:

   Frequency + order tracking = correct first unique [OK]

Hint: Track order with queue alongside frequency map [OK]

Common Mistakes:

• Sorting string loses original order
• Using vector loses quick lookup
• Stopping count too early