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diff --git a/works/life/operating-system-challenge/3/README.md b/works/life/operating-system-challenge/3/README.md
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@@ -0,0 +1,5 @@
+![problem](problem.jpg)
+
+Translation: Use a lock to achieve that 3 threads print A, B, C in turn for 10 times. Output is `ABCABC...`
+
+Code is in `main.cpp`.
diff --git a/works/life/operating-system-challenge/3/main.cpp b/works/life/operating-system-challenge/3/main.cpp
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+++ b/works/life/operating-system-challenge/3/main.cpp
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+#include <iostream>
+#include <mutex>
+#include <thread>
+
+int main() {
+  int turn = 1;
+  int count = 0;
+  std::mutex mutex;
+
+  auto thread_proc1 = [&] {
+    while (true) {
+      {
+        std::lock_guard<std::mutex> guard(mutex);
+        if (count >= 10)
+          break;
+        if (turn == 1) {
+          std::cout << "A" << std::endl;
+          turn = 2;
+        }
+      }
+
+      std::this_thread::yield();
+    }
+  };
+
+  auto thread_proc2 = [&] {
+    while (true) {
+      {
+        std::lock_guard<std::mutex> guard(mutex);
+        if (count >= 10)
+          break;
+        if (turn == 2) {
+          std::cout << "B" << std::endl;
+          turn = 3;
+        }
+      }
+
+      std::this_thread::yield();
+    }
+  };
+
+  auto thread_proc3 = [&] {
+    while (true) {
+      {
+        std::lock_guard<std::mutex> guard(mutex);
+        if (count >= 10)
+          break;
+        if (turn == 3) {
+          std::cout << "C" << std::endl;
+          turn = 1;
+          count++;
+        }
+      }
+
+      std::this_thread::yield();
+    }
+  };
+
+  std::thread thread1(thread_proc1);
+  std::thread thread2(thread_proc2);
+  std::thread thread3(thread_proc3);
+
+  thread1.join();
+  thread2.join();
+  thread3.join();
+
+  return 0;
+}
diff --git a/works/life/operating-system-challenge/3/problem.jpg b/works/life/operating-system-challenge/3/problem.jpg
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+++ b/works/life/operating-system-challenge/3/problem.jpg