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

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

@@ -43,5 +43,15 @@
  */
 
 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
 }