diff --git a/Encode and Decode Strings/note.md b/Encode and Decode Strings/note.md
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+++ b/Encode and Decode Strings/note.md	
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+# 方法論
+
+## Non ascii value
+
+インプットされる値はASCIIのみなので、delimeterとしてnon-ascii値を採用する方法。（ex: π, あ)
+
+時間計算量: o(n)
+空間計算量: o(k)
+
+## Escaping
+
+エスケーピングを使用して、delimeterが元の文字列か判別がつくようにする手法。
+
+- https://en.wikipedia.org/wiki/Escape_sequence
+
+時間計算量: o(n)
+空間計算量: o(k)
+
+エスケープ関連
+- [double-terminated-string](https://devblogs.microsoft.com/oldnewthing/20091008-00/?p=16443)
+- [Protocol Buffers](https://ja.wikipedia.org/wiki/Protocol_Buffers), [例](https://github.com/protocolbuffers/protobuf/blob/1194440c2489fc58051a245b5db74c0fd1bbf4b0/upb/json/decode.c#L356)
+
+## Chunked transfer encoding
+
+もとの文字列の長さとdelimeterを使用して、chunk毎にもとの文字列長さがわかるようencodeする方法。
+
+- https://en.wikipedia.org/wiki/Chunked_transfer_encoding
+
+時間計算量: o(n)
+空間計算量: o(k)
+
+
+#　振り返り
+
+## Step1
+
+値がnon-asciiであるため、non-ascii値を使用してjoin,splitを使用としたが、スペースもascii値であることに気づいておらず失敗。
+
+```step1.py
+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        return ' '.join(strs)
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        if s == '':
+            return [""]
+        else:
+            return s.split()
+
+
+# Your Codec object will be instantiated and called as such:
+# codec = Codec()
+# codec.decode(codec.encode(strs))
+```
+
diff --git a/Encode and Decode Strings/step1.py b/Encode and Decode Strings/step1.py
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+++ b/Encode and Decode Strings/step1.py	
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+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        return ' '.join(strs)
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        if s == '':
+            return [""]
+        else:
+            return s.split()
+
+
+# Your Codec object will be instantiated and called as such:
+# codec = Codec()
+# codec.decode(codec.encode(strs))
diff --git a/Encode and Decode Strings/step2-using-chunked-transfer-encoding.py b/Encode and Decode Strings/step2-using-chunked-transfer-encoding.py
new file mode 100644
index 0000000..187d2b3
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+++ b/Encode and Decode Strings/step2-using-chunked-transfer-encoding.py	
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+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        encoded_string = ""
+
+        for s in strs:
+            encoded_string += str(len(s)) + "/:" + s
+
+        return encoded_string
+
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        decoded_strings = []
+        i = 0
+
+        while i < len(s):
+            delimeter = s.find("/:", i)
+            word_length = int(s[i:delimeter])
+            string = s[delimeter+2 : delimeter+2+word_length]
+            decoded_strings.append(string)
+            i = delimeter + 2 + word_length
+        
+        return decoded_strings
diff --git a/Encode and Decode Strings/step2-using-escape.py b/Encode and Decode Strings/step2-using-escape.py
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+++ b/Encode and Decode Strings/step2-using-escape.py	
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+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        res = ""
+
+        for s in strs:
+            res += s.replace("/", "//") + "/:"
+        return res
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        res = []
+        current_string = ""
+
+        i = 0
+
+        while i < len(s):
+            if s[i:i+2] == "/:":
+                res.append(current_string)
+                current_string = ""
+                i += 2
+            elif s[i:i+2] == "//":
+                current_string += "/"
+                i += 2
+            else:
+                current_string += s[i]
+                i += 1
+        
+        return res
+
+
+# Your Codec object will be instantiated and called as such:
+# codec = Codec()
+# codec.decode(codec.encode(strs))
diff --git a/Encode and Decode Strings/step2-using-non-ascii-char.py b/Encode and Decode Strings/step2-using-non-ascii-char.py
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index 0000000..c730fce
--- /dev/null
+++ b/Encode and Decode Strings/step2-using-non-ascii-char.py	
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+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        return "あ".join(strs)
+        
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        return s.split('あ')
+
+
+# Your Codec object will be instantiated and called as such:
+# codec = Codec()
+# codec.decode(codec.encode(strs))
diff --git a/Encode and Decode Strings/step2-using-non-ascii-delimeter.cpp b/Encode and Decode Strings/step2-using-non-ascii-delimeter.cpp
new file mode 100644
index 0000000..f1fdca1
--- /dev/null
+++ b/Encode and Decode Strings/step2-using-non-ascii-delimeter.cpp	
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+class Codec {
+public:
+
+    // Encodes a list of strings to a single string.
+    string encode(vector<string>& strs) {
+        string res = "";
+        for (int i = 0; i < strs.size(); i++) {
+            string str = strs[i];
+            res += str + "あ";
+        }
+        return res;
+    }
+
+    // Decodes a single string to a list of strings.
+    vector<string> decode(string s) {
+        vector<string> res;
+        int pos = 0;
+        while (pos < s.size()) {
+            pos = s.find("あ");
+            res.push_back(s.substr(0,pos));
+            s.erase(0, pos+3);
+        }
+        return res;
+    }
+};
+
+// Your Codec object will be instantiated and called as such:
+// Codec codec;
+// codec.decode(codec.encode(strs));
diff --git a/Encode and Decode Strings/step3.py b/Encode and Decode Strings/step3.py
new file mode 100644
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+++ b/Encode and Decode Strings/step3.py	
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+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        encoded_string = ""
+
+        for s in strs:
+            encoded_string += str(len(s)) + "#" + s
+        
+        return encoded_string
+        
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        decoded_strings = []
+        i = 0
+
+        while i < len(s):
+            delim = s.find("#", i)
+            word_length = int(s[i:delim])
+            decoded_str = s[delim+1 : delim + 1 + word_length]
+            decoded_strings.append(decoded_str)
+            i = delim + 1 + word_length
+        
+        return decoded_strings
diff --git a/Encode and Decode Strings/step4.py b/Encode and Decode Strings/step4.py
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index 0000000..fbef891
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+++ b/Encode and Decode Strings/step4.py	
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+class Codec:
+    def encode(self, strs: List[str]) -> str:
+        """Encodes a list of strings to a single string.
+        """
+        encoded_string = ""
+
+        for s in strs:
+            encoded_string += s.replace('/', '//') + '/:'
+        
+        return encoded_string
+
+    def decode(self, s: str) -> List[str]:
+        """Decodes a single string to a list of strings.
+        """
+        decoded_strings = []
+        current_string = ""
+        i = 0
+        
+        while i < len(s):
+            if s[i:i+2] == "/:":
+                decoded_strings.append(current_string)
+                current_string = ""
+                i += 2
+            elif s[i:i+2] == "//":
+                current_string += "/"
+                i += 2
+            else:
+                current_string += s[i]
+                i += 1
+        
+        return decoded_strings