diff --git a/arai60/atoi/phase1.py b/arai60/atoi/phase1.py
new file mode 100644
index 0000000..c25bb25
--- /dev/null
+++ b/arai60/atoi/phase1.py
@@ -0,0 +1,71 @@
+INT_MAX = 2 ** 31 -1
+INT_MIN = - 2 ** 31
+# 仮に数字の場合文字列の最初に来うるのは符号か数字のみ
+# 符号が来た場合も次の文字が数字じゃなければならない
+# is_number -> extract_number(integer部分を抜き取る) -> 数字化処理
+
+def char_is_number(char: str)->bool:
+    if ord(char) >= ord("0") and ord("9") >= ord(char):
+        return True
+    return False
+
+def is_number(s: str)->bool:
+    for index, char in enumerate(s):
+        if char == " ":
+            continue
+        elif char == "-" or char == "+":
+            if index == len(s) - 1:
+                #符号が終端の場合
+                return False
+            else:
+                #次の文字が数字かどうかで判断
+                return char_is_number(s[index+1])
+        elif char_is_number(char):
+            return True
+        else:
+            return False
+    return False
+
+def search_start_index(s: str)->int:
+    for index, char in enumerate(s):
+        if char == "+" or char == "-":
+            return index
+        elif char_is_number(char):
+            return index
+
+def extract_number(s: str)->str:
+    start_index = search_start_index(s)
+    end_index = len(s)
+    for index in range(start_index+1, len(s)):
+        if not char_is_number(s[index]):
+            end_index = index
+            break
+    return s[start_index:end_index]
+
+def string_to_integer(s: str)->int:
+    if not is_number(s):
+        return 0
+
+    extracted_number_string = extract_number(s)
+    sign = 1
+    if extracted_number_string[0] == "-":
+        sign = -1
+        extracted_number_string = extracted_number_string[1:]
+    elif extracted_number_string[0] == "+":
+        extracted_number_string = extracted_number_string[1:]
+    
+    after_atoi_number = 0
+    for index, char in enumerate(extracted_number_string):
+        digit = len(extracted_number_string) - index - 1
+        after_atoi_number += (ord(char)-ord("0")) * 10 ** digit
+    
+    return after_atoi_number * sign
+    
+
+
+def clamp(number: int) -> int:
+    if number > INT_MAX:
+        return INT_MAX
+    elif number < INT_MIN:
+        return INT_MIN
+    return number
diff --git a/arai60/atoi/phase2.py b/arai60/atoi/phase2.py
new file mode 100644
index 0000000..21bba53
--- /dev/null
+++ b/arai60/atoi/phase2.py
@@ -0,0 +1,54 @@
+# https://github.com/cheeseNA/leetcode/pull/5/files
+# まず空白に関してカーソルを動かす, そのあと符号に関してカーソルを動かし, 数字が続く限りwhileで処理をしていた。
+# 僕のphase1の解答と違いif文に関しての複雑な制御がいらなくて賢かった。(でもatoiを作れという課題なのにpythonのintを使ってるのはどうなんだろう)
+
+# https://github.com/usatie/leetcode/commit/9b4ea58330b31c3c1371fe242bc02ab1a9012329
+# 上のほぼC++版, 上もだがオーバーフローは最後に確認ではなく逐次確認の方が良いのだと思った
+# 最後に確認する方法だと下手したらオーバーフローが二周してくる場合とかもあるなと思った
+
+INT_MAX = 2 ** 31 -1
+INT_MIN = - 2 ** 31
+
+def char_is_number(char: str)->bool:
+    if ord(char) >= ord("0") and ord("9") >= ord(char):
+        return True
+    return False
+
+def string_to_integer(s: str)->int:
+    if len(s) == 0:
+        return 0
+
+    current_pointer = 0
+    while current_pointer < len(s):
+        # 空白に関して処理をする
+        if s[current_pointer] != " ":
+            break
+        current_pointer += 1
+    if current_pointer == len(s):
+        return 0
+    
+    sign = 1
+    #符号処理
+    if s[current_pointer] == "+":
+        current_pointer += 1
+    elif s[current_pointer] == "-":
+        sign = -1
+        current_pointer += 1
+    
+    abs_value = 0
+    while current_pointer < len(s) and char_is_number(s[current_pointer]):
+        abs_value *= 10
+        abs_value += ord(s[current_pointer]) - ord("0")
+        if abs_value * sign > INT_MAX:
+            return INT_MAX
+        elif abs_value * sign < INT_MIN:
+            return INT_MIN
+        current_pointer += 1
+    
+    return abs_value * sign
+
+
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        after_atoi_number = string_to_integer(s)
+        return after_atoi_number
\ No newline at end of file
diff --git a/arai60/atoi/phase3.py b/arai60/atoi/phase3.py
new file mode 100644
index 0000000..3ab68c4
--- /dev/null
+++ b/arai60/atoi/phase3.py
@@ -0,0 +1,44 @@
+INT_MAX = 2 ** 31 - 1
+INT_MIN = - 2 ** 31
+def char_is_number(char: str)->int:
+    if ord(char) >= ord("0") and ord("9") >= ord(char):
+        return True
+    return False
+
+def string_to_integer(s: str)->int:
+    if len(s) == 0:
+        return 0
+
+    current_pointer = 0
+    while current_pointer < len(s):
+        if s[current_pointer] != " ":
+            break
+        current_pointer += 1
+
+    if current_pointer == len(s):
+        return 0
+    
+    sign = 1
+    # 符号処理
+    if s[current_pointer] == "+":
+        current_pointer += 1
+    elif s[current_pointer] == "-":
+        sign = -1
+        current_pointer += 1
+    
+    abs_value = 0
+    while current_pointer < len(s) and char_is_number(s[current_pointer]):
+        abs_value *= 10
+        abs_value += ord(s[current_pointer]) - ord("0")
+        if abs_value * sign > INT_MAX:
+            return INT_MAX
+        elif abs_value * sign < INT_MIN:
+            return INT_MIN
+        current_pointer += 1
+
+    return abs_value * sign
+
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        after_atoi_number = string_to_integer(s)
+        return after_atoi_number
\ No newline at end of file
diff --git a/arai60/atoi/phase4.py b/arai60/atoi/phase4.py
new file mode 100644
index 0000000..18ad7ed
--- /dev/null
+++ b/arai60/atoi/phase4.py
@@ -0,0 +1,34 @@
+INT_MAX = 2 ** 31 - 1
+INT_MIN = - 2 ** 31
+
+def char_is_number(c: str)->bool:
+    return ord("0") <= ord(c) <= ord("9")
+
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        index = 0
+        while index < len(s) and s[index] == " ":
+            index += 1
+        
+        if index == len(s):
+            return 0
+        
+        sign = 1
+        if s[index] == "+":
+            index += 1
+        elif s[index] == "-":
+            sign = -1
+            index += 1
+        
+        absolute_value = 0
+        while index < len(s) and char_is_number(s[index]):
+            absolute_value *= 10
+            absolute_value += ord(s[index]) - ord("0")
+            if absolute_value * sign > INT_MAX:
+                return INT_MAX
+            elif absolute_value * sign < INT_MIN:
+                return INT_MIN
+            index += 1
+
+        return absolute_value * sign
+        
\ No newline at end of file