282. First Character That Appears Once
Core1000 ms256 MBSolved by 0%
Print the first character of the string that appears exactly once. If every
character repeats, print None.
leetcode -> l
aabb -> None
Two passes. Count every character first, then walk the string again and return the first whose count is 1.
Both passes matter. Counting alone does not tell you which came first, and walking alone does not tell you what repeats later.
Constraints - `1 ≤ length ≤ 200000` - The string contains lowercase letters only.
Input
A single line containing a string of lowercase letters.
Output
Print the first non-repeating character, or None if there is none.
Inputleetcode
Outputl
Notehas its answer as the very first character
Inputaabb
OutputNone
Notehas every character repeating
Hints
Four rungs, in order. The last two open once you have submitted an attempt. A wrong one counts.
Hint 1Where to start
Hint 2The approachOpen hint 1 first. This one carries on from it.
Hint 3PseudocodeOpen hint 2 first. This one carries on from it.
Hint 4Full solutionOpen hint 3 first. This one carries on from it.