Update 0017.电话号码的字母组合.md

problems/0017.电话号码的字母组合.md

@@ -281,7 +281,40 @@ class Solution {
 }
 ```
 
-Python：
+Python3：
+```Python3
+class Solution:
+    ans = []
+    s = ''
+    letterMap = {
+        '2': 'abc',
+        '3': 'def',
+        '4': 'ghi',
+        '5': 'jkl',
+        '6': 'mno',
+        '7': 'pqrs',
+        '8': 'tuv',
+        '9': 'wxyz'
+    }
+
+    def letterCombinations(self, digits):
+        self.ans.clear()
+        if digits == '':
+            return self.ans
+        self.backtracking(digits, 0)
+        return self.ans
+
+    def backtracking(self, digits, index):
+        if index == len(digits):
+            self.ans.append(self.s)
+            return
+        else:
+            letters = self.letterMap[digits[index]]  # 取出数字对应的字符集
+            for letter in letters:
+                self.s = self.s + letter  # 处理
+                self.backtracking(digits, index + 1)
+                self.s = self.s[:-1]      # 回溯
+```
 
 
 Go：