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. Example 1:Input: s = "abccccdd"Output: 7Explanation: One longest palindrome that can be built is "dccaccd", whose length is 7.Example 2:Input: s = "a"Output: 1Explanation: The longest palindrome that can be built is "a", whose length is 1. Constraints:1 <= s.length <= 2000s consists of lowercase and/or uppercase English letters only.

To solve this problem, we can use a hash map to count the frequency of each character in the string. Then, we iterate through the hash map to add the count of each character to the length of the longest palindrome. If the count of a character is even, we can add the whole count to the length. If the count is odd, we can add the count minus one to the length. After the iteration, if the length of the string is greater than the length of the longest palindrome, we can add one to the length of the longest palindrome because we can put a character with odd count in the middle of the palindrome. Here are the steps:

1. Initialize a hash map to count the frequency of each character.
2. Iterate through the string to count the frequency of each character.
3. Initialize a variable length to store the length of the longest palindrome.
4. Iterate through the hash map. For each entry, if the count is even, add the count to length. If the count is odd, add the count minus one to length.
5. After the iteration, if the length of the string is greater than length, add one to length.
6. Return length.

This algorithm works because a palindrome can be built by pairing characters. If a character has an even count, we can pair all of them. If a character has an odd count, we can pair all but one of them. The remaining unpaired character can be put in the middle of the palindrome.