Create 0409 Longest Palindrome.md

Author: jjjllxx

greedy/0409 Longest Palindrome.md

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+# 409. Longest Palindrome
+https://leetcode-cn.com/problems/longest-palindrome/  
+Given a string s which consists of lowercase or uppercase letters, return the length of the longest palindrome that can be built with those letters.  
+Letters are case sensitive, for example, "Aa" is not considered a palindrome here.   
+
+Example 1:  
+Input: s = "abccccdd"  
+Output: 7  
+Explanation:  
+One longest palindrome that can be built is "dccaccd", whose length is 7.  
+
+Example 2:  
+Input: s = "a"  
+Output: 1  
+
+Example 3:  
+Input: s = "bb"  
+Output: 2  
+
+Constraints:  
+1 <= s.length <= 2000  
+s consists of lowercase and/or uppercase English letters only.  
+
+``` python3
+class Solution:
+    def longestPalindrome(self, s: str) -> int:
+        d=collections.Counter(s)
+        ans=0
+        for k in d:
+            ans+=d[k]//2*2
+        if ans<len(s):
+            ans+=1
+        return ans
+```