diff --git a/problems/matrix-diagonal-sum.cpp b/problems/matrix-diagonal-sum.cpp
new file mode 100644
index 0000000..5433d8c
--- /dev/null
+++ b/problems/matrix-diagonal-sum.cpp
@@ -0,0 +1,46 @@
+#include <cassert>
+#include <vector>
+
+class Solution {
+  struct indices {
+    int x;
+    int y;
+
+    bool operator==(const indices& other) const = default;
+    indices& operator+=(const indices& other) {
+      x += other.x;
+      y += other.y;
+      return *this;
+    }
+
+    int operator[](const std::vector<std::vector<int>>& mat) const {
+      return mat[y][x];
+    }
+  };
+
+ public:
+  int diagonalSum(const std::vector<std::vector<int>>& mat) {
+    int sum = 0;
+
+    indices down{0, 0}, up{0, static_cast<int>(mat.size()) - 1};
+    indices d_down{1, 1}, d_up{1, -1};
+    for (std::size_t i = 0; i < mat.size(); ++i, down += d_down, up += d_up) {
+      sum += down[mat];
+      if (down != up) {
+        sum += up[mat];
+      }
+    }
+
+    return sum;
+  }
+};
+
+int main() {
+  Solution s;
+
+  assert((s.diagonalSum({{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}) == 25));
+  assert((s.diagonalSum(
+              {{1, 1, 1, 1}, {1, 1, 1, 1}, {1, 1, 1, 1}, {1, 1, 1, 1}}) == 8));
+  assert((s.diagonalSum({{5}}) == 5));
+  return 0;
+}