Move all solutions into separate directory (cleanup)

the beginning

solutions/reverse-integer/Solution.java

@@ -0,0 +1,25 @@
+// Accepted
+// Runtime: 1 ms
+// Memory Usage: 35.8 MB
+// Submitted: January 14th, 2021
+
+class Solution {
+    public int reverse(int x) {
+        // Remember whether it was negative or not
+        boolean negative = x < 0;
+        x = Math.abs(x);
+        int reversed = 0;
+        
+        int degree = (int) Math.floor(Math.log10(x));
+        
+        while (x > 0) {
+            // digit x 10^degree
+            reversed += Math.pow(10, degree--) * (x % 10);
+            x /= 10;
+        }
+        
+        if (reversed == Integer.MAX_VALUE || reversed == Integer.MIN_VALUE)
+            return 0;
+        return negative ? -reversed : reversed;
+    }
+}