feat(problems): add implementation for first unique character in a string

work/Easy/Hash Table/387.first-unique-character-in-a-string.go

@@ -40,8 +40,18 @@
  * 	• 1 <= s.length <= 10^5
  *
  * 	• s consists of only lowercase English letters.
-*/
+ */
 
 func firstUniqChar(s string) int {
+	freq := map[rune]int{}
+	for _, c := range s {
+		freq[c]++
+	}
 
+	for i, c := range s {
+		if freq[c] == 1 {
+			return i
+		}
+	}
+	return -1
 }