diff --git a/exercises/06_loop/main.cpp b/exercises/06_loop/main.cpp
index 0b01455a..3c04483c 100644
--- a/exercises/06_loop/main.cpp
+++ b/exercises/06_loop/main.cpp
@@ -3,7 +3,7 @@
 // TODO: 改正函数实现，实现正确的缓存优化斐波那契计算
 static unsigned long long fibonacci(int i) {
     // TODO: 为缓存设置正确的初始值
-    static unsigned long long cache[128], cached = 2;
+    static unsigned long long cache[128], cached;
     // TODO: 设置正确的循环条件
     for (; false; ++cached) {
         cache[cached] = cache[cached - 1] + cache[cached - 2];
@@ -11,9 +11,16 @@ static unsigned long long fibonacci(int i) {
     return cache[i];
 }
 
+// ---- 不要修改以下代码 ----
 int main(int argc, char **argv) {
+    ASSERT(fibonacci(0) == 0, "fibonacci(0) should be 0");
+    ASSERT(fibonacci(1) == 1, "fibonacci(1) should be 1");
+    ASSERT(fibonacci(2) == 1, "fibonacci(2) should be 1");
+    ASSERT(fibonacci(3) == 2, "fibonacci(3) should be 2");
+    ASSERT(fibonacci(10) == 55, "fibonacci(10) should be 55");
+
     auto fib100 = fibonacci(100);
-    ASSERT(fib100 == 3736710778780434371, "fibonacci(100) should be 3736710778780434371");
     std::cout << "fibonacci(100) = " << fib100 << std::endl;
+    ASSERT(fib100 == 3736710778780434371, "fibonacci(100) should be 3736710778780434371");
     return 0;
 }
diff --git a/exercises/15_class_derive/main.cpp b/exercises/15_class_derive/main.cpp
index ea653a01..fa3faec0 100644
--- a/exercises/15_class_derive/main.cpp
+++ b/exercises/15_class_derive/main.cpp
@@ -4,9 +4,11 @@
 
 // 三个类型的定义在下方，它们的关系是：B 派生自 A 并包含一个 X 类型的成员。
 
+// ↓↓↓ 这是声明
 struct X;
 struct A;
 struct B;
+// ↑↑↑ 这是声明
 
 int main(int argc, char **argv) {
     X x = X(1);
@@ -34,6 +36,8 @@ int main(int argc, char **argv) {
     return 0;
 }
 
+// ↓↓↓ 这是定义
+
 struct X {
     int x;
 
diff --git a/exercises/23_std_vector/main.cpp b/exercises/23_std_vector/main.cpp
index c6b48172..8358a9d1 100644
--- a/exercises/23_std_vector/main.cpp
+++ b/exercises/23_std_vector/main.cpp
@@ -8,6 +8,7 @@ int main(int argc, char **argv) {
     {
         std::vector<int> vec{1, 2, 3, 4, 5};
         ASSERT(vec.size() == ?, "Fill in the correct value.");
+        // THINK: `std::vector` 的大小是什么意思？与什么有关？
         ASSERT(sizeof(vec) == ?, "Fill in the correct value.");
         int ans[]{1, 2, 3, 4, 5};
         ASSERT(std::memcmp(vec.?, ans, sizeof(ans)) == 0, "Fill in the correct values.");
diff --git a/exercises/27_std_map/main.cpp b/exercises/27_std_map/main.cpp
index 98a2f0b0..fcccca34 100644
--- a/exercises/27_std_map/main.cpp
+++ b/exercises/27_std_map/main.cpp
@@ -1,8 +1,37 @@
 #include "../exercise.h"
 #include <map>
-#include <unordered_map>
 
+// READ: `std::map` <https://zh.cppreference.com/w/cpp/container/map>
+// READ: `std::unordered_map` <https://zh.cppreference.com/w/cpp/container/unordered_map>
+
+template<class k, class v>
+bool key_exists(std::map<k, v> const &map, k const &key) {
+    // TODO: 实现函数
+}
+
+template<class k, class v>
+void set(std::map<k, v> &map, k key, v value) {
+    // TODO: 实现函数
+}
+
+// ---- 不要修改以下代码 ----
 int main(int argc, char **argv) {
-    ASSERT(false, "todo!");
+    using namespace std::string_literals;
+
+    std::map<std::string, std::string> secrets;
+
+    set(secrets, "hello"s, "world"s);
+    ASSERT(key_exists(secrets, "hello"s), "\"hello\" shoud be in `secrets`");
+    ASSERT(!key_exists(secrets, "foo"s), "\"foo\" shoud not be in `secrets`");
+
+    set(secrets, "foo"s, "bar"s);
+    set(secrets, "Infini"s, "Tensor"s);
+    ASSERT(secrets["hello"] == "world", "hello -> world");
+    ASSERT(secrets["foo"] == "bar", "foo -> bar");
+    ASSERT(secrets["Infini"] == "Tensor", "Infini -> Tensor");
+
+    set(secrets, "hello"s, "developer"s);
+    ASSERT(secrets["hello"] == "developer", "hello -> developer");
+
     return 0;
 }
diff --git a/exercises/28_std_transform/main.cpp b/exercises/28_std_transform/main.cpp
index a94bcedb..f4dc25a5 100644
--- a/exercises/28_std_transform/main.cpp
+++ b/exercises/28_std_transform/main.cpp
@@ -1,7 +1,20 @@
 #include "../exercise.h"
 #include <algorithm>
+#include <string>
+#include <vector>
+
+// READ: `std::transform` <https://zh.cppreference.com/w/cpp/algorithm/transform>
+// READ: `std::vector::begin` <https://zh.cppreference.com/w/cpp/container/vector/begin>
 
 int main(int argc, char **argv) {
-    ASSERT(false, "todo!");
+    std::vector<int> val{8, 13, 21, 34, 55};
+    // TODO: 调用 `std::transform`，将 `v` 中的每个元素乘以 2，并转换为字符串，存入 `ans`
+    // std::vector<std::string> ans
+    ASSERT(ans.size() == val.size(), "ans size should be equal to val size");
+    ASSERT(ans[0] == "16", "ans[0] should be 16");
+    ASSERT(ans[1] == "26", "ans[1] should be 26");
+    ASSERT(ans[2] == "42", "ans[2] should be 42");
+    ASSERT(ans[3] == "68", "ans[3] should be 68");
+    ASSERT(ans[4] == "110", "ans[4] should be 110");
     return 0;
 }
diff --git a/exercises/29_std_accumulate/main.cpp b/exercises/29_std_accumulate/main.cpp
index ac240036..7d9a8da7 100644
--- a/exercises/29_std_accumulate/main.cpp
+++ b/exercises/29_std_accumulate/main.cpp
@@ -1,7 +1,13 @@
 #include "../exercise.h"
 #include <numeric>
 
+// READ: `std::accumulate` <https://zh.cppreference.com/w/cpp/algorithm/accumulate>
+
 int main(int argc, char **argv) {
-    ASSERT(false, "todo!");
+    using DataType = float;
+    int shape[]{1, 3, 224, 224};
+    // TODO: 调用 `std::accumulate` 计算 `shape` 的元素之积
+    // int size =
+    ASSERT(size = 602112, "4x1x3x224x224 = 602112");
     return 0;
 }