First Non Repeating Character Using Hash in DSA C# - Time & Space Complexity

Time Complexity: First Non Repeating Character Using Hash

O(n)

Understanding Time Complexity

We want to know how long it takes to find the first character that appears only once in a string using a hash map.

How does the time needed grow as the string gets longer?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public char FirstNonRepeatingChar(string s) {
    var counts = new Dictionary<char, int>();
    foreach (var c in s) {
        counts[c] = counts.TryGetValue(c, out var cnt) ? cnt + 1 : 1;
    }
    foreach (var c in s) {
        if (counts[c] == 1) return c;
    }
    return '\0';
}
    

This code counts how many times each character appears, then finds the first character with count one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Two loops over the string characters.
• How many times: Each loop runs once over all characters, so twice in total.

How Execution Grows With Input

As the string length grows, the number of operations grows roughly twice as fast because of two passes.

Input Size (n)	Approx. Operations
10	About 20 operations
100	About 200 operations
1000	About 2000 operations

Pattern observation: Operations grow linearly with input size.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the first non-repeating character grows directly in proportion to the string length.

Common Mistake

[X] Wrong: "Because there are two loops, the time complexity is O(n²)."

[OK] Correct: The loops run one after another, not nested, so their times add up, not multiply.

Interview Connect

Understanding how to analyze simple loops and hash map usage helps you explain efficient solutions clearly in interviews.

Self-Check

"What if we used a nested loop to check each character against all others instead of a hash map? How would the time complexity change?"

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