Move all solutions into separate directory (cleanup)

the beginning

solutions/first-unique-character-in-a-string/Solution.java

@@ -0,0 +1,22 @@
+// Accepted
+// Runtime: 7 ms
+// Memory Usage: 39.6 MB
+// Submitted: January 14th, 2021
+
+class Solution {
+    public int firstUniqChar(String s) {
+        int[] count = new int[26]; //
+        int n = s.length();
+        
+        // Count the appearances of each character
+        for (int i = 0; i < n; i++)
+            count[s.charAt(i) - 'a']++;
+        
+        // Iterate only the array again, return the index of the character that only appeared once
+        for (int i = 0; i < n; i++)
+            if (count[s.charAt(i) - 'a'] == 1)
+                return i;
+                
+        return -1;
+    }
+}