add 409

Author: JackieHoo

205.md

@@ -1 +1,6 @@
 # Isomorphic Strings
+### 解法一
+
+```go
+
+```

409.md

@@ -1 +1,23 @@
 # Longest Palindrome
+
+### 解法一
+```go
+func longestPalindrome(s string) int {
+    if len(s) == 0 { // 判断零值
+        return 0
+    }
+    result := 0
+    mi := make(map[rune]int) // 使用额外的数据结构
+    for _,v := range s {
+        mi[v]++
+    }
+
+    for  _,v := range mi {
+        result += (v/2)*2// 无论是奇数既或者是偶数用 v/2 * 2 都是取出来的偶数个。
+    }
+    if result < len(s) { // 就是偶数个都找到了，如果有多的几个奇数，那么取出来一个放到中间即可。
+        result +=1
+    }
+    return result
+}
+```

README.md

@@ -5,7 +5,7 @@ All leetcode questions that solved in different methods.
 |question number|URL|type|result|
 |:---:|:---:|:---:|:---:|
 |[9](./9.md)|[Palindrome Number](https://leetcode-cn.com/problems/palindrome-number/description/)|string|😯|
-|[205](./205.md)|[Isomorphic Strings](https://leetcode-cn.com/problems/isomorphic-strings/description/)|string|😯|
+|[205](./205.md)|[Isomorphic Strings](https://leetcode-cn.com/problems/isomorphic-strings/description/)|string|👌|
 |[240](./240.md)|[Search a 2D Matrix II](https://leetcode-cn.com/problems/search-a-2d-matrix-ii/description/)|array|👌|
 |[242](./242.md)|[Valid Anagram](https://leetcode-cn.com/problems/valid-anagram/description/)|string|👌|
 |[283](./283.md)|[Move Zeroes](https://leetcode-cn.com/problems/move-zeroes/description/)|array|👌|