Merge branch 'master' into master

problems/0017.电话号码的字母组合.md

@@ -1,10 +1,10 @@
 <p align="center">
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-<p align="center"><strong>欢迎大家参与本项目，贡献其他语言版本的代码，拥抱开源，让更多学习算法的小伙伴们收益！</strong></p>
+<p align="center"><strong>欢迎大家<a href="https://mp.weixin.qq.com/s/tqCxrMEU-ajQumL1i8im9A">参与本项目</a>，贡献其他语言版本的代码，拥抱开源，让更多学习算法的小伙伴们收益！</strong></p>
 
 
 # 17.电话号码的字母组合
@@ -272,7 +272,7 @@ class Solution {
         String str = numString[digits.charAt(num) - '0'];
         for (int i = 0; i < str.length(); i++) {
             temp.append(str.charAt(i));
-            //回溯
+            //c
             backTracking(digits, numString, num + 1);
             //剔除末尾的继续尝试
             temp.deleteCharAt(temp.length() - 1);
@@ -282,6 +282,7 @@ class Solution {
 ```
 
 Python：
+
 ```Python
 class Solution:
     ans = []
@@ -316,6 +317,28 @@ class Solution:
                 self.s = self.s[:-1]      # 回溯
 ```
 
+python3：
+
+```python3
+class Solution:
+    def letterCombinations(self, digits: str) -> List[str]:
+        self.s = ""
+        res = []
+        letterMap = ["","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"]
+        if len(digits) == 0: return res
+        def backtrack(digits,index):
+            if index == len(digits):
+                return res.append(self.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)
+        return res
+```
+
 
 Go：