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

Recall & Review

beginner

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

Use a hash map 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 using a hash map to find the first non-repeating character?

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

beginner

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

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

beginner

What should be returned if all characters in the string repeat?

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

intermediate

How does the hash map help improve efficiency compared to checking each character against all others?

It reduces time complexity from O(n²) to O(n) by counting once and then checking counts.

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

A O(log n)

B O(n)

C O(n²)

D O(1)

Using a hash map, we count characters in one pass and find the first unique in another, both O(n).

What does the hash map store in this problem?

A Indexes as keys and characters as values

B Characters as keys and their indexes as values

C Characters as keys and their counts as values

D Only unique characters

The hash map stores each character with how many times it appears.

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

A "a"

B "c"

C "b"

D -1

All characters repeat, so no non-repeating character exists.

Why do we need to check characters in the original order after counting?

A To find the first character with count one

B To sort the characters

C To remove duplicates

D To reverse the string

We check in order to find the first character that appears only once.

Which data structure is NOT suitable for counting characters efficiently?

A Linked list

B Array (for fixed charset)

C Dictionary

D Hash map

Linked list is inefficient for counting because it requires searching for each character.

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

Think about counting first, then searching.

What are the advantages of using a hash map for this problem compared to nested loops?

Focus on speed and simplicity.

Practice

1. What does the "First Non Repeating Character Using Hash" algorithm primarily do?

easy

A. Reverses the string using a hash map

B. Counts the total number of characters in a string

C. Sorts characters of a string in alphabetical order

D. Finds the first character in a string that appears only once

Solution

1. Step 1: Understand the problem goal

   The algorithm aims to find the first character in the string that does not repeat anywhere else.

2. Step 2: Role of hash map

   A hash map stores character counts to quickly check which characters appear once.

3. Final Answer:

   Finds the first character in a string that appears only once -> Option D

4. Quick Check:

   First non repeating character = Finds the first character in a string that appears only once [OK]

Hint: Look for the first unique character using counts [OK]

Common Mistakes:

• Thinking it counts total characters only
• Confusing sorting with finding unique character
• Assuming it reverses the string

2. Which Java collection is best suited to store character counts for this problem?

easy

A. ArrayList<Character>

B. HashMap

C. LinkedList<Character>

D. TreeSet<Character>

Solution

1. Step 1: Identify data structure needs

   We need to map each character to its count, so a key-value structure is required.

2. Step 2: Choose appropriate Java collection

   HashMap stores characters as keys and their counts as values efficiently.

3. Final Answer:

   HashMap -> Option B

4. Quick Check:

   Use HashMap for character counts [OK]

Hint: Use HashMap for key-value pairs of char and count [OK]

Common Mistakes:

• Using list which doesn't map keys to counts
• Using TreeSet which stores unique sorted elements only
• Using LinkedList which is not key-value based

3. What is the output of the following Java code snippet?

String s = "swiss";
HashMap map = new HashMap<>();
for(char c : s.toCharArray()) {
  map.put(c, map.getOrDefault(c, 0) + 1);
}
for(char c : s.toCharArray()) {
  if(map.get(c) == 1) {
    System.out.println(c);
    break;
  }
}

medium

A. w

B. s

C. i

D. No output

Solution

1. Step 1: Count character frequencies

   Characters and counts: s=3, w=1, i=1.

2. Step 2: Find first character with count 1

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

3. Final Answer:

   w -> Option A

4. Quick Check:

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

Hint: Check counts then first unique char in order [OK]

Common Mistakes:

• Choosing 's' which repeats
• Choosing 'i' which is unique but appears after 'w'
• Assuming no output if not careful

4. Identify the error in this code snippet for finding the first non repeating character:

String s = "hello";
HashMap map = new HashMap<>();
for(int i = 0; i < s.length(); i++) {
  map.put(s.charAt(i), 1);
}
for(int i = 0; i < s.length(); i++) {
  if(map.get(s.charAt(i)) == 1) {
    System.out.println(s.charAt(i));
    break;
  }
}

medium

A. The map.put always sets count to 1, ignoring repeats

B. The loop should start from 1, not 0

C. The map should use Character keys, not int

D. The break statement is missing

Solution

1. Step 1: Analyze map population logic

   The code sets count to 1 for every character without incrementing, so repeats are not counted.

2. Step 2: Effect on finding non repeating character

   Since all counts are 1, the code incorrectly treats all characters as unique.

3. Final Answer:

   The map.put always sets count to 1, ignoring repeats -> Option A

4. Quick Check:

   Count update missing causes wrong results [OK]

Hint: Increment count instead of setting 1 every time [OK]

Common Mistakes:

• Not incrementing counts in map
• Wrong loop indices
• Confusing key types in map

5. Given the string "aabbccddeeffg", which character will be the first non repeating character using the hash method? What if the string is "aabbccddeeffggh"?

hard

A. First: g, Second: g

B. First: a, Second: h

C. First: g, Second: h

D. First: h, Second: h

Solution

1. Step 1: Analyze first string "aabbccddeeffg"

   All characters except 'g' appear twice; 'g' appears once, so first non repeating is 'g'.

2. Step 2: Analyze second string "aabbccddeeffggh"

   Now 'g' appears twice, 'h' appears once, so first non repeating is 'h'.

3. Final Answer:

   First: g, Second: h -> Option C

4. Quick Check:

   Check counts carefully for each string [OK]

Hint: Check counts carefully; last unique changes with input [OK]

Common Mistakes:

• Assuming first character is always unique
• Ignoring updated counts in second string
• Confusing repeated characters as unique