diff --git a/include/s0404_sum_of_left_leaves.hpp b/include/s0404_sum_of_left_leaves.hpp
new file mode 100644
index 0000000..4dacbe5
--- /dev/null
+++ b/include/s0404_sum_of_left_leaves.hpp
@@ -0,0 +1,15 @@
+#ifndef S0404_sum_of_left_leaves_HPP
+#define S0404_sum_of_left_leaves_HPP
+
+#include <queue>
+
+#include "structures.hpp"
+
+using namespace std;
+
+class S0404 {
+ public:
+  int sumOfLeftLeaves(TreeNode* root);
+};
+
+#endif
diff --git a/notes/src/btree_iter.md b/notes/src/btree_iter.md
index cc72bb0..2882afa 100644
--- a/notes/src/btree_iter.md
+++ b/notes/src/btree_iter.md
@@ -159,3 +159,5 @@ vector<vector<int>> iter(Node *root) {
   return result;
 }
 ```
+
+如果需要找某一层的什么节点的话，考虑用这个版本的层序遍历。
diff --git a/src/s0404_sum_of_left_leaves.cpp b/src/s0404_sum_of_left_leaves.cpp
new file mode 100644
index 0000000..f9e7f08
--- /dev/null
+++ b/src/s0404_sum_of_left_leaves.cpp
@@ -0,0 +1,27 @@
+#include "s0404_sum_of_left_leaves.hpp"
+
+struct TreeNodeState_S0404 {
+  TreeNode* ptr;
+  int sum;
+  bool isLeft;
+};
+
+int S0404::sumOfLeftLeaves(TreeNode* root) {
+  int sum{0};
+  queue<TreeNodeState_S0404> q;
+  if (root) q.push(TreeNodeState_S0404{root, root->val, false});
+  while (!q.empty()) {
+    TreeNodeState_S0404 front = q.front();
+    q.pop();
+    if (front.ptr->left == nullptr && front.ptr->right == nullptr &&
+        front.isLeft)
+      sum += front.ptr->val;
+    if (front.ptr->left)
+      q.push(TreeNodeState_S0404{front.ptr->left, front.sum + front.ptr->left->val,
+                           true});
+    if (front.ptr->right)
+      q.push(TreeNodeState_S0404{front.ptr->right, front.sum + front.ptr->right->val,
+                           false});
+  }
+  return sum;
+}