Merge branch 'master' of github.com:JalanJiang/leetcode-notebook

Author: JalanJiang

docs/data-structure/string/README.md

@@ -1423,6 +1423,49 @@ class Solution(object):
         return result
 ```
 
+## 771. 宝石与石头
+
+[原题链接](https://leetcode-cn.com/problems/jewels-and-stones/)
+
+### 解法一：遍历字符串
+
+遍历字符串，寻找 S 中存在于 J 中的字母个数。
+
+```python
+class Solution:
+    def numJewelsInStones(self, J: str, S: str) -> int:
+        res = 0
+        for j in J:
+            for s in S:
+                if j == s:
+                    res += 1
+        return res
+```
+
+pythonic 一点：
+
+```python
+class Solution:
+    def numJewelsInStones(self, J: str, S: str) -> int:
+        return len([x for x in S if x in J])
+```
+
+### 解法二：哈希表
+
+记录 `S` 中每个字母出现的次数，再把 `J` 中存在的字母次数都相加。
+
+```python
+class Solution:
+    def numJewelsInStones(self, J: str, S: str) -> int:
+        s_count = dict()
+        for s in S:
+            s_count[s] = s_count.get(s, 0) + 1
+        res = 0
+        for j in J:
+            res += s_count.get(j, 0)
+        return res
+```
+
 ## 1108. IP 地址无效化
 
 [原题链接](https://leetcode-cn.com/problems/defanging-an-ip-address/)