Move all solutions into separate directory (cleanup)

the beginning

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

@@ -0,0 +1,24 @@
+// Accepted
+// Runtime: 23 ms
+// Memory Usage: 39.5 MB
+// Submitted: January 14th, 2021
+
+class Solution {
+    public int firstUniqChar(String s) {
+        Map<Character, Integer> count = new HashMap<Character, Integer>();
+        
+        // Count the number of appearances of each character
+        for (int i = 0; i < s.length(); i++) {
+            char val = s.charAt(i);
+            count.put(val, count.getOrDefault(val, 0) + 1);
+        }
+        
+        // Iterate along the array again, and find the first character that only appears once
+        for (int i = 0; i < s.length(); i++) {
+            if (count.get(s.charAt(i)) == 1)
+                return i;
+        }
+        
+        return -1;
+    }
+}