Move all solutions into separate directory (cleanup)

the beginning

solutions/two-sum/Solution.java

@@ -0,0 +1,18 @@
+// Accepted
+// Runtime: 0 ms
+// Memory Usage: 39.7 MB
+// Submitted: January 12th, 2021
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        Map<Integer, Integer> map = new HashMap<>();
+        for (int i = 0; i < nums.length; i++) {
+            int complement = target - nums[i];
+            if (map.containsKey(complement)) {
+                return new int[] { map.get(complement), i };
+            }
+            map.put(nums[i], i);
+        }
+        throw new IllegalArgumentException("No two sum solution");
+    }
+}