Update 0017.电话号码的字母组合.md

self.s 语法纠正

problems/0017.电话号码的字母组合.md

@@ -322,20 +322,20 @@ python3：
 ```py
 class Solution:
     def letterCombinations(self, digits: str) -> List[str]:
-        self.s = ""
         res = []
+        s = ""
         letterMap = ["","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"]
-        if len(digits) == 0: return res
-        def backtrack(digits,index):
+        if not len(digits): return res
+        def backtrack(digits,index, s):
             if index == len(digits):
-                return res.append(self.s)
+                return res.append(s)
             digit = int(digits[index])  #将index指向的数字转为int
             letters = letterMap[digit]  #取数字对应的字符集
             for i in range(len(letters)):
-                self.s += letters[i]
-                backtrack(digits,index + 1)  #递归，注意index+1，一下层要处理下一个数字
-                self.s = self.s[:-1]  #回溯
-        backtrack(digits,0)
+                s += letters[i]
+                backtrack(digits, index+1, s)  #递归，注意index+1，一下层要处理下一个数字
+                s = s[:-1]  #回溯
+        backtrack(digits, 0, s)
         return res
 ```