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Mar 21, 2022
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docs: add a description of the solution to lc problem: No.0409 #763

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13 changes: 13 additions & 0 deletions solution/0400-0499/0409.Longest Palindrome/README.md
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<!-- 这里可写通用的实现逻辑 -->

> 一个回文字符串，最多存在一个出现奇数次数的字符，

先统计所有字符出现的次数，通用的方式是哈希表。题目已说明只存在大小写字母（52 种可能），也可以使用数组来存储。

而后，可分两种方式：

- 布尔变量
- 累加出现次数为偶数的数值。
- 对于奇数，第一次出现，完整累加；后续出现，则需要对次数 `-1` 去奇，再累加。
- 计数器
- 记录奇数出现的次数，最后的结果回文长度由 `s.length - count` 得知。
- 如果只存在一个奇数，那么可以直接返回 `s.length`.

<!-- tabs:start -->

### **Python3**
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