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Algorithm Challenge Available At CodeFights

Note: Write a solution that only iterates over the string once and uses O(1) additional memory, since this is what you would be asked to do during a real interview.

Given a string s, find and return the first instance of a non-repeating character in it. If there is no such character, return '_'.

Example

• For s = "abacabad", the output should be firstNotRepeatingCharacter(s) = 'c'.

There are 2 non-repeating characters in the string: 'c' and 'd'. Return c since it appears in the string first.

• For s = "abacabaabacaba", the output should be firstNotRepeatingCharacter(s) = '_'.

There are no characters in this string that do not repeat.

Hints

• split()
• hasOwnProperty()

Input/Output

• [execution time limit] 5 seconds (ts)

• [input] string s A string that contains only lowercase English letters.

Guaranteed constraints:

1 ≤ s.length ≤ 10^5.

• [output] char The first non-repeating character in s, or '_' if there are no characters that do not repeat.