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

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

@@ -2,46 +2,56 @@
  * 387. First Unique Character in a String
  * Difficulty: Easy
  * https://leetcode.com/problems/first-unique-character-in-a-string/
- * 
+ *
  * ──────────────────────────────────────────────────
- * 
+ *
  * Given a string s, find the first non-repeating character in it and
  * return its index. If it does not exist, return -1.
- * 
- *  
- * 
+ *
+ *
+ *
  * Example 1:
- * 
+ *
  * Input: s = "leetcode"
- * 
+ *
  * Output: 0
- * 
+ *
  * Explanation:
- * 
+ *
  * The character 'l' at index 0 is the first character that does not
  * occur at any other index.
- * 
+ *
  * Example 2:
- * 
+ *
  * Input: s = "loveleetcode"
- * 
+ *
  * Output: 2
- * 
+ *
  * Example 3:
- * 
+ *
  * Input: s = "aabb"
- * 
+ *
  * Output: -1
- * 
- *  
- * 
+ *
+ *
+ *
  * Constraints:
- * 
+ *
  * 	• 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
 }