Add Dart solution for Two Sum

dart/two_sum.dart

@@ -0,0 +1,22 @@
+/*
+Given an array of integers nums and an integer target, return 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.
+*/
+
+class Solution {
+  List<int> twoSum(List<int> nums, int target) {
+    final Map<int, int> seen = {};
+
+    for (int i = 0; i < nums.length; i++) {
+      final int complement = target - nums[i];
+      if (seen.containsKey(complement)) {
+        return [seen[complement]!, i];
+      }
+
+      seen[nums[i]] = i;
+    }
+
+    return [];
+  }
+}