387. First Unique Character in a String

https://leetcode.com/problems/first-unique-character-in-a-string/

Given a string, find the first non-repeating character in it and return it's index. If it doesn't exist, return -1.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode",
return 2.

Note: You may assume the string contain only lowercase letters.
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First unique character means we have to check all characters, time complexity is linear

Time - O(n)

Space - O(1)