105. find the first duplicate character in a string

easy - accepted / - tried

Given a string which might have duplicate letters, write a function to find the first duplicate.

firstDuplicate('abca')// 'a'firstDuplicate('abcdefe')// 'e'firstDuplicate('abcdef')// null

What is the time & space cost of your approach ? Can you do better?

Always try to find a better approach.