Merge pull request #1870 from Undertone0809/patch-7

Update 添加电话号码字母组合Python新方法

problems/0017.电话号码的字母组合.md

@@ -354,6 +354,28 @@ class Solution:
         for letter in letters:
             self.backtracking(digits, index + 1, answer + letter)    # 递归至下一层 + 回溯
 ```
+**使用itertools**
+```python
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
+        import itertools
+        if not digits:
+            return list()
+        
+        phoneMap = {
+            "2": "abc",
+            "3": "def",
+            "4": "ghi",
+            "5": "jkl",
+            "6": "mno",
+            "7": "pqrs",
+            "8": "tuv",
+            "9": "wxyz",
+        }
+
+        groups = (phoneMap[digit] for digit in digits)
+        return ["".join(combination) for combination in itertools.product(*groups)]
+```
 
 
 ## Go