diff --git a/0111-minimum-depth-of-binary-tree/memo.md b/0111-minimum-depth-of-binary-tree/memo.md
new file mode 100644
index 0000000..286f2e5
--- /dev/null
+++ b/0111-minimum-depth-of-binary-tree/memo.md
@@ -0,0 +1,11 @@
+### step1
+
+maximum depth of binary treeを参考に、左や右のnodeがない場合の処理だけ書いたら解けた。
+
+### step2
+
+変更点なし。
+
+### step3
+
+3回通すまで書き直し。
diff --git a/0111-minimum-depth-of-binary-tree/step1.cpp b/0111-minimum-depth-of-binary-tree/step1.cpp
new file mode 100644
index 0000000..42ac17b
--- /dev/null
+++ b/0111-minimum-depth-of-binary-tree/step1.cpp
@@ -0,0 +1,21 @@
+class Solution {
+public:
+    int minDepth(TreeNode* root) {
+        if (root == nullptr) {
+            return 0;
+        }
+
+        int leftDepth = minDepth(root->left);
+        int rightDepth = minDepth(root->right);
+
+        if (leftDepth == 0) {
+            return rightDepth + 1;
+        }
+
+        if (rightDepth == 0) {
+            return leftDepth + 1;
+        }
+
+        return min(leftDepth, rightDepth) + 1;
+    }
+};
diff --git a/0111-minimum-depth-of-binary-tree/step2.cpp b/0111-minimum-depth-of-binary-tree/step2.cpp
new file mode 100644
index 0000000..42ac17b
--- /dev/null
+++ b/0111-minimum-depth-of-binary-tree/step2.cpp
@@ -0,0 +1,21 @@
+class Solution {
+public:
+    int minDepth(TreeNode* root) {
+        if (root == nullptr) {
+            return 0;
+        }
+
+        int leftDepth = minDepth(root->left);
+        int rightDepth = minDepth(root->right);
+
+        if (leftDepth == 0) {
+            return rightDepth + 1;
+        }
+
+        if (rightDepth == 0) {
+            return leftDepth + 1;
+        }
+
+        return min(leftDepth, rightDepth) + 1;
+    }
+};
diff --git a/0111-minimum-depth-of-binary-tree/step3.cpp b/0111-minimum-depth-of-binary-tree/step3.cpp
new file mode 100644
index 0000000..42ac17b
--- /dev/null
+++ b/0111-minimum-depth-of-binary-tree/step3.cpp
@@ -0,0 +1,21 @@
+class Solution {
+public:
+    int minDepth(TreeNode* root) {
+        if (root == nullptr) {
+            return 0;
+        }
+
+        int leftDepth = minDepth(root->left);
+        int rightDepth = minDepth(root->right);
+
+        if (leftDepth == 0) {
+            return rightDepth + 1;
+        }
+
+        if (rightDepth == 0) {
+            return leftDepth + 1;
+        }
+
+        return min(leftDepth, rightDepth) + 1;
+    }
+};