First Non-Repeating Character in a String

Question

## Problem

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

## Example
```
"leetcode" -> 0; "aabb" -> -1
```

## Constraints
- 1 ≤ s.length ≤ 10^5

## Approach
Frequency map in one pass, then a second pass for the first count-1 char.