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

Recall & Review

beginner

What is the purpose of using a hash (dictionary) in finding the first non-repeating character?

A hash (dictionary) stores the count of each character in the string, allowing quick lookup to identify characters that appear only once.

beginner

How do you identify the first non-repeating character after counting frequencies?

By scanning the string from left to right and checking the count of each character in the hash, the first character with count 1 is the first non-repeating character.

intermediate

Why is the time complexity of this approach O(n)?

Because we traverse the string twice: once to count characters and once to find the first non-repeating character, each operation is O(n), so total is O(n).

beginner

What happens if all characters repeat in the string?

If all characters repeat, the function returns a special value (like -1 or None) indicating no non-repeating character exists.

beginner

Show the Python code snippet to count characters using a dictionary.

counts = {} for ch in s: counts[ch] = counts.get(ch, 0) + 1

What data structure is best to count character frequencies for this problem?

A Stack

B Queue

C Hash (dictionary)

D Linked List

A hash (dictionary) allows fast counting and lookup of character frequencies.

What is the first step in finding the first non-repeating character?

A Remove duplicates

B Sort the string

C Reverse the string

D Count frequency of each character

Counting frequencies helps identify which characters appear only once.

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

A 'a'

B -1 or None

C 'b'

D 'c'

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

What is the time complexity of this approach?

A O(n)

B O(n^2)

C O(log n)

D O(1)

Two passes over the string make the time complexity linear, O(n).

Which of these is NOT required to find the first non-repeating character?

A Sorting the string

B Traversing the string

C Counting character frequency

D Checking counts in hash

Sorting is not needed; it changes the order and is unnecessary.

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

Think about counting first, then scanning.

Describe the time complexity and why this method is efficient.

Focus on number of traversals and hash operations.