First Non Repeating Character Using Hash in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Find the first non-repeating character index

What is the output of this code snippet that finds the index of the first non-repeating character in the string?

DSA Python

def first_non_repeating_char(s):
    count = {}
    for ch in s:
        count[ch] = count.get(ch, 0) + 1
    for i, ch in enumerate(s):
        if count[ch] == 1:
            return i
    return -1

print(first_non_repeating_char("swiss"))

A 1

B 2

C 3

D -1

💡 Hint

Check the count of each character and find the first with count 1.

The function counts each character's occurrences. 's' appears multiple times, 'w' appears once at index 1, the first character 's' repeats. The first non-repeating character is 'w' at index 1. The code returns the index of the first character with count 1, which is index 1.

The string is "swiss". Characters and counts: s:3, w:1, i:1. The first non-repeating character is 'w' at index 1. So output is 1.

❓ Predict Output

intermediate

Output of first non-repeating character index for 'leetcode'

What is the output of this code that finds the first non-repeating character index in the string 'leetcode'?

DSA Python

def first_non_repeating_char(s):
    count = {}
    for ch in s:
        count[ch] = count.get(ch, 0) + 1
    for i, ch in enumerate(s):
        if count[ch] == 1:
            return i
    return -1

print(first_non_repeating_char("leetcode"))

A 2

B -1

C 0

D 1

💡 Hint

Look for the first character that appears only once.

Counting characters in "leetcode": l:1, e:3, t:1, c:1, o:1, d:1.

First character 'l' appears once at index 0.

The correct output is 0.

🔧 Debug

advanced

Identify the error in this code for first non-repeating character

What error does this code raise when run?

DSA Python

def first_non_repeating_char(s):
    count = {}
    for ch in s:
        count[ch] += 1
    for i, ch in enumerate(s):
        if count[ch] == 1:
            return i
    return -1

print(first_non_repeating_char("apple"))

A KeyError

B TypeError

C IndexError

D No error, output 0

💡 Hint

Check how the dictionary is updated before keys exist.

The code tries to increment count[ch] without initializing it first.

Since count is empty, count[ch] += 1 causes a KeyError on first occurrence.

🧠 Conceptual

advanced

Why use a hash map for first non-repeating character?

Why is a hash map (dictionary) the best data structure to find the first non-repeating character in a string?

A It uses less memory than arrays or lists for counting characters.

B It sorts the characters automatically to find the first unique one.

C It stores characters in order and removes duplicates instantly.

D It allows counting character occurrences in O(n) time and O(1) average lookup.

💡 Hint

Think about counting and quick lookup.

A hash map lets us count how many times each character appears in one pass.

Then we can check each character in order to find the first with count 1.

This approach is efficient and simple.

❓ Predict Output

expert

Output of code with Unicode characters

What is the output of this code that finds the first non-repeating character index in the Unicode string 'aáa'?

DSA Python

def first_non_repeating_char(s):
    count = {}
    for ch in s:
        count[ch] = count.get(ch, 0) + 1
    for i, ch in enumerate(s):
        if count[ch] == 1:
            return i
    return -1

print(first_non_repeating_char('aáa'))

A 0

B 1

C -1

D 2

💡 Hint

Unicode characters are distinct; count each separately.

The string is 'aáa'.

Characters: 'a' appears twice (index 0 and 2), 'á' appears once at index 1.

The first non-repeating character is 'á' at index 1.