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diff --git a/top-interview-questions/easy/array/09_two_sum.cc b/top-interview-questions/easy/array/09_two_sum.cc
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+++ b/top-interview-questions/easy/array/09_two_sum.cc
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+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        // When you find, e.g., 2, map (2 -> idx 0) for the event
+        // in which you find the 7.
+        std::vector<int> pair(2);
+        std::unordered_map<int, int> val_to_idx;
+
+        for (size_t i = 0; i < nums.size(); ++i) {
+            const int other = target - nums[i];
+            const auto other_idx = val_to_idx.find(other);
+
+            if (other_idx != val_to_idx.end()) {
+                pair[0] = i;
+                pair[1] = other_idx->second;
+                break;
+            }
+
+            val_to_idx[nums[i]] = i;
+        }
+
+        return pair;
+    }
+};