First Letter to Appear Twice

CMakeLists.txt

@@ -227,6 +227,7 @@ add_task(find-the-student-that-will-replace-the-chalk)
 add_task(find-the-town-judge)
 add_task(find-the-winner-of-the-circular-game)
 add_task(first-bad-version)
+add_task(first-letter-to-appear-twice)
 add_task(first-missing-positive)
 add_task(first-unique-character-in-a-string)
 add_task(fizz-buzz)

solutions/first-letter-to-appear-twice/CMakeLists.txt

@@ -0,0 +1 @@
+add_catch(test_first_letter_to_appear_twice test.cpp)

solutions/first-letter-to-appear-twice/solution.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+class Solution {
+public:
+  static char repeatedCharacter(std::string s) {
+    std::vector<int> counter(128);
+    for (auto c : s) {
+      if (counter[c]++) {
+        return c;
+      }
+    }
+    throw;
+  }
+};

solutions/first-letter-to-appear-twice/test.cpp

@@ -0,0 +1,26 @@
+#include <catch.hpp>
+
+#include <solution.hpp>
+
+TEST_CASE("Simple") {
+  struct TestCase {
+    std::string s;
+    char expected;
+  };
+
+  std::vector<TestCase> test_cases{
+      {
+          .s = "abccbaacz",
+          .expected = 'c',
+      },
+      {
+          .s = "abcdd",
+          .expected = 'd',
+      },
+  };
+
+  for (const auto &[s, expected] : test_cases) {
+    const auto actual = Solution::repeatedCharacter(s);
+    REQUIRE(expected == actual);
+  }
+}