diff --git a/problems/404-Sum-of-Left-Leaves/CMakeLists.txt b/problems/404-Sum-of-Left-Leaves/CMakeLists.txt
new file mode 100644
index 0000000..5f85c55
--- /dev/null
+++ b/problems/404-Sum-of-Left-Leaves/CMakeLists.txt
@@ -0,0 +1,14 @@
+add_executable(404 main.cpp kernel.cpp)
+
+if(T AND EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/test.cpp)
+    add_executable(404_test kernel.cpp test.cpp)
+    target_link_libraries(404_test gtest_main)
+    # add_test(NAME 4041 COMMAND 4041_test)    
+    include(GoogleTest)
+    gtest_discover_tests(404_test)
+else(T AND EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/test.cpp)
+    message(STATUS "Test building is skipped for 404")
+endif(T AND EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/test.cpp)
+
+
+install(TARGETS 404 DESTINATION ${INSTALL_DIR})
diff --git a/problems/404-Sum-of-Left-Leaves/kernel.cpp b/problems/404-Sum-of-Left-Leaves/kernel.cpp
new file mode 100644
index 0000000..caab460
--- /dev/null
+++ b/problems/404-Sum-of-Left-Leaves/kernel.cpp
@@ -0,0 +1,45 @@
+#include "kernel.h"
+#include <queue>
+using namespace std;
+
+int Solution::sumOfLeftLeaves(TreeNode* root)
+{
+    int sum = 0;
+    queue<TreeNode*> q;     // BFS
+    bool isLeft = true;
+
+    // root
+    if (root == nullptr)
+        return 0;
+    if (root->left == nullptr && root->right == nullptr)
+        return 0;
+    else 
+    {
+        q.push(root->left);
+        q.push(root->right);
+    }
+
+    while (!q.empty())
+    {
+        TreeNode *cur = q.front();
+        if (cur != nullptr)
+        {
+            if (cur->left == nullptr && cur->right == nullptr)
+            {
+                if (isLeft)
+                    sum += cur->val;
+            }
+            else
+            {
+                q.push(cur->left);
+                q.push(cur->right);
+            }
+            
+        }
+        
+        q.pop();
+        isLeft = !isLeft;
+    }
+
+    return sum;
+}
\ No newline at end of file
diff --git a/problems/404-Sum-of-Left-Leaves/kernel.h b/problems/404-Sum-of-Left-Leaves/kernel.h
new file mode 100644
index 0000000..08f7005
--- /dev/null
+++ b/problems/404-Sum-of-Left-Leaves/kernel.h
@@ -0,0 +1,16 @@
+#include "utils/binary_tree.h"
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    static int sumOfLeftLeaves(TreeNode* root);
+};
diff --git a/problems/404-Sum-of-Left-Leaves/main.cpp b/problems/404-Sum-of-Left-Leaves/main.cpp
new file mode 100644
index 0000000..966b480
--- /dev/null
+++ b/problems/404-Sum-of-Left-Leaves/main.cpp
@@ -0,0 +1,24 @@
+#include "kernel.h"
+#include <iostream>
+#include <vector>
+using namespace std;
+
+int main()
+{
+    int size;
+    cout << "Input <size>: ";
+    while (cin >> size)
+    {
+        vector<int> nums(size);
+        cout << "Input <nums>...: ";
+        for (int i = 0; i < size; i++)
+            cin >> nums[i];
+
+        BinaryTree bt;
+        TreeNode* root = bt.construct_from_level_order_array(nums.data(), size);
+        int ret = Solution::sumOfLeftLeaves(root);
+        printf("%d\n", ret);
+
+        cout << "Input <size>: ";
+    }
+}
\ No newline at end of file
diff --git a/problems/404-Sum-of-Left-Leaves/test.cpp b/problems/404-Sum-of-Left-Leaves/test.cpp
new file mode 100644
index 0000000..697bf7e
--- /dev/null
+++ b/problems/404-Sum-of-Left-Leaves/test.cpp
@@ -0,0 +1,32 @@
+#include "kernel.h"
+#include <gtest/gtest.h>
+#include <vector>
+using namespace std;
+
+TEST(_404, _1)
+{
+    vector<int> nums{3, 9, 20, -1, -1, 15, 7};
+    int expect = 24;
+
+    BinaryTree bt;
+    TreeNode *root = bt.construct_from_level_order_array(nums.data(), nums.size());
+    EXPECT_EQ(Solution::sumOfLeftLeaves(root), expect);
+}
+
+TEST(_404, _2)
+{
+    vector<int> nums{3};
+    int expect = 0;
+
+    BinaryTree bt;
+    TreeNode *root = bt.construct_from_level_order_array(nums.data(), nums.size());
+    EXPECT_EQ(Solution::sumOfLeftLeaves(root), expect);
+}
+
+TEST(_404, _3)
+{
+    int expect = 0;
+
+    BinaryTree bt;
+    EXPECT_EQ(Solution::sumOfLeftLeaves(nullptr), expect);
+}
\ No newline at end of file