First Unique Character

Difficulty: 1000 2000 ms 128 MB

strings

Print the first character that appears exactly once in S, or NONE if every character repeats.

Input: one line with the string S.

Output: the first non-repeating character, or NONE.

Constraints:

• 1 ≤ |S| ≤ 10⁴
• S contains lowercase English letters

Sample Tests

swiss

w

aabb

NONE

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