First Non-Repeating Character in a String

Problem

Return the index of the first character that does not repeat; return -1 if none.

Input / Output

• Input: a string s.
• Output: the index of the first non-repeating character, or -1 if every character repeats.

Constraints

• 1 ≤ s.length ≤ 10^5

Example

"leetcode" -> 0; "aabb" -> -1