cpp: add “645. Set Mismatch”

Signed-off-by: Matej Focko <mfocko@redhat.com>

cpp/set-mismatch.cpp

@@ -0,0 +1,33 @@
+#include <cassert>
+#include <unordered_set>
+#include <vector>
+
+class Solution {
+  public:
+    std::vector<int> findErrorNums(const std::vector<int> &nums) {
+        int expected_sum = nums.size() * (1 + nums.size()) / 2;
+        int sum = 0;
+        int unique_sum = 0;
+
+        std::unordered_set<int> seen;
+        seen.reserve(nums.size());
+        for (const int x : nums) {
+            sum += x;
+            if (!seen.contains(x)) {
+                unique_sum += x;
+            }
+            seen.insert(x);
+        }
+
+        return {sum - unique_sum, expected_sum - unique_sum};
+    }
+};
+
+int main() {
+    Solution s;
+
+    assert((s.findErrorNums(std::vector{1, 2, 2, 4}) == std::vector{2, 3}));
+    assert((s.findErrorNums(std::vector{1, 1}) == std::vector{1, 2}));
+
+    return 0;
+}