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

Recall & Review

beginner

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

Use a hash (like an array or dictionary) 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 finding the first non-repeating character using a hash?

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

beginner

In C, what data structure is commonly used as a hash to count characters for ASCII strings?

An integer array of size 256 (for all ASCII characters) is used to store counts indexed by character ASCII values.

intermediate

What is the time complexity of finding the first non-repeating character using a hash in a string of length n?

O(n) because we scan the string twice, but each scan is linear time.

beginner

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

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

What does the hash store when finding the first non-repeating character?

A Index of each character

B Count of each character

C Characters sorted alphabetically

D Length of the string

The hash stores how many times each character appears.

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

A Four times

B Once

C Three times

D Twice

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

What size array is typically used as a hash for ASCII characters in C?

A 128

B 1024

C 256

D 512

256 covers all ASCII characters.

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

A -1

B 1

C 0

D 2

All characters repeat, so return -1.

What is the time complexity of this method?

A O(n)

B O(n log n)

C O(n^2)

D O(1)

Two linear scans make it O(n).

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

Think about counting first, then checking order.

Describe the advantages of using a hash to find the first non-repeating character compared to checking each character one by one.

Focus on efficiency and speed.

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 reverse the string quickly

B. To sort the characters alphabetically

C. To remove duplicate characters from the string

D. To count the frequency of each character efficiently

Solution

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

   A hash (dictionary) stores characters as keys and their counts as values, allowing quick frequency counting.

2. Step 2: Identify why frequency counting helps

   Knowing how many times each character appears helps 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 D

4. Quick Check:

   Hash counts characters = frequency count [OK]

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

Common Mistakes:

• Thinking hash sorts characters
• Assuming hash reverses string
• Using hash to remove duplicates only

2. Which of the following C code snippets correctly initializes a hash map (using an array) to count characters for ASCII strings?

easy

A. int count[256] = {0};

B. int count[] = NULL;

C. int count[256]; // uninitialized

D. int count = 256;

Solution

1. Step 1: Check correct array initialization

   In C, to count ASCII chars, an int array of size 256 initialized to zero is needed: int count[256] = {0};

2. Step 2: Identify incorrect options

   int count[] = NULL; sets pointer to NULL, no array. int count[256]; // uninitialized is uninitialized, may contain garbage. int count = 256; is a single int, not array.

3. Final Answer:

   int count[256] = {0}; -> Option A

4. Quick Check:

   Array size 256 with zero init = correct hash map [OK]

Hint: Use int count[256] = {0} for ASCII char frequency [OK]

Common Mistakes:

• Not initializing array to zero
• Using pointer instead of array
• Declaring single int instead of array

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

char str[] = "swiss";
int count[256] = {0};
for (int i = 0; str[i] != '\0'; i++) {
    count[(int)str[i]]++;
}
for (int i = 0; str[i] != '\0'; i++) {
    if (count[(int)str[i]] == 1) {
        printf("%c", str[i]);
        break;
    }
}

medium

A. s

B. i

C. w

D. No output

Solution

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

   Characters: s(3 times), w(1 time), i(1 time)

2. Step 2: Find first character with count 1 in order

   Order: s (count 3), w (count 1) -> first non-repeating is 'w'

3. Final Answer:

   w -> Option C

4. Quick Check:

   First unique char in "swiss" = w [OK]

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

Common Mistakes:

• Printing first char without checking count
• Confusing 'i' and 'w' counts
• Ignoring character order

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

char str[] = "hello";
int count[256];
for (int i = 0; str[i] != '\0'; i++) {
    count[(int)str[i]]++;
}
for (int i = 0; str[i] != '\0'; i++) {
    if (count[(int)str[i]] == 1) {
        printf("%c", str[i]);
        break;
    }
}

medium

A. count array is not initialized to zero

B. Loop condition is incorrect

C. Casting char to int is wrong

D. printf format specifier is incorrect

Solution

1. Step 1: Check count array initialization

   count array is declared but not initialized, so it contains garbage values causing wrong counts.

2. Step 2: Explain impact of uninitialized array

   Incrementing garbage values leads to incorrect frequency counts, so first non-repeating char cannot be found correctly.

3. Final Answer:

   count array is not initialized to zero -> Option A

4. Quick Check:

   Uninitialized count array causes wrong frequency [OK]

Hint: Always initialize count array to zero before counting [OK]

Common Mistakes:

• Forgetting to initialize count array
• Assuming default zero initialization
• Misusing loop conditions

5. Given the string "aabbccddeffg", which character will be identified as the first non-repeating character using a hash map counting method? Consider the code below:

char str[] = "aabbccddeffg";
int count[256] = {0};
for (int i = 0; str[i] != '\0'; i++) {
    count[(int)str[i]]++;
}
for (int i = 0; str[i] != '\0'; i++) {
    if (count[(int)str[i]] == 1) {
        printf("%c", str[i]);
        break;
    }
}

hard

A. g

B. e

C. d

D. f

Solution

1. Step 1: Count frequencies of each character

   Characters and counts: a(2), b(2), c(2), d(2), e(1), f(2), g(1)

2. Step 2: Find first character with count 1 in order

   Order: a,b,c,d,e,f,g; e and g have count 1, but e appears before g.

3. Step 3: Check carefully the string order

   String is "aabbccddeffg"; 'e' appears once at position 8, 'g' at position 11. So first non-repeating is 'e'.

4. Final Answer:

   e -> Option B

5. Quick Check:

   First unique char in string = e [OK]

Hint: Check counts and order carefully to find first unique char [OK]

Common Mistakes:

• Choosing last unique char instead of first
• Ignoring character order in string
• Miscounting character frequencies