Added Problem 1

0001_Two_Sum.java

@@ -0,0 +1,20 @@
+// id: 1
+// Name: Two Sum
+// link: https://leetcode.com/problems/two-sum/
+// Difficulty: Easy
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        HashMap<Integer, Integer> map = new HashMap<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            if (map.containsKey(target-nums[i])) {
+                return new int[] {map.get(target-nums[i]), i};
+            }
+            else {
+                map.put(nums[i], i);
+            }
+        }
+        return null;
+    }
+}