diff --git a/Problems/Arrays/TwoSum.java b/Problems/Arrays/TwoSum.java
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+++ b/Problems/Arrays/TwoSum.java
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+/**
+ * Problem: Two Sum (Issue #12)
+                      *
+    * Given an array of integers nums and an integer target, return the indices
+    * of the two numbers such that they add up to target.
+    * You may assume that each input would have exactly one solution, and you
+    * may not use the same element twice.
+    *
+    * Approach: HashMap for O(n) lookup
+    * - Iterate through the array
+    * - For each element, check if (target - element) exists in the map
+    * - If yes, return the stored index and the current index
+    * - Otherwise, store the element and its index in the map
+    *
+    * Time Complexity: O(n)
+    * Space Complexity: O(n)
+    *
+    * Test Cases:
+  * nums = [2, 7, 11, 15], target = 9 -> [0, 1]
+    * nums = [3, 2, 4], target = 6 -> [1, 2]
+    * nums = [3, 3], target = 6 -> [0, 1]
+    */
+
+import java.util.Arrays;
+import java.util.HashMap;
+
+public class TwoSum {
+
+    public int[] twoSum(int[] nums, int target) {
+              // Map stores: value -> index
+          HashMap<Integer, Integer> map = new HashMap<>();
+
+          for (int i = 0; i < nums.length; i++) {
+                        int complement = target - nums[i];
+
+                  if (map.containsKey(complement)) {
+                                    // Found the pair
+                            return new int[]{map.get(complement), i};
+                  }
+
+                  // Store current element with its index
+                  map.put(nums[i], i);
+          }
+
+          // Problem guarantees a solution exists, so this won't be reached
+          throw new IllegalArgumentException("No two sum solution found.");
+    }
+
+    public static void main(String[] args) {
+              TwoSum solution = new TwoSum();
+
+          // Test case 1: Expected [0, 1]
+          System.out.println(Arrays.toString(solution.twoSum(new int[]{2, 7, 11, 15}, 9)));
+
+          // Test case 2: Expected [1, 2]
+          System.out.println(Arrays.toString(solution.twoSum(new int[]{3, 2, 4}, 6)));
+
+          // Test case 3: Expected [0, 1]
+          System.out.println(Arrays.toString(solution.twoSum(new int[]{3, 3}, 6)));
+    }
+}