Find the first character in a string that appears exactly once.
Input
A single line: a string s of lower-case English letters.
Output
The first character that occurs exactly once, or none if every character repeats.
First Non-Repeating Character
Mediumstringshashing
Examples
Example 1
- Input
leetcode
- Output
l
l is the first letter that appears only once.
Example 2
- Input
aabbcc
- Output
none
Every letter repeats, so none.
Example 3
- Input
abcabd
- Output
c
a and b repeat; c is the first singleton.
Constraints
- 1 <= length of s <= 10^5
- s consists of lower-case English letters.
Hints
Reveal hint 1
First count how many times each character appears.
Reveal hint 2
Then scan the string left to right and return the first character whose count is 1.
Reference solution (spoiler — try it yourself first)
Approach
Count character frequencies in one pass, then scan again for the first character with frequency 1.
from collections import Counter
s = input()
c = Counter(s)
print(next((ch for ch in s if c[ch] == 1), "none"))
Bring us the problem, not a perfect specification
Tell us what needs to change, who it affects and any important deadline. We will review the context and reply with useful next questions.
- 01Share contextDescribe the workflow, constraint or risk.
- 02Clarify togetherWe identify missing facts and useful options.
- 03Choose a startAgree a focused assessment or delivery step.