409 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
Explanation: The longest palindrome that can be built is "a", whose length is 1.

Constraints:

• 1 <= s.length <= 2000

• s consists of lowercase and/or uppercase English letters only.

一道简单题想了我半天没想出来。原来是贪心。一开始以为只是判断偶数的个数，然后最后如果有基数的，就result + 1。后来看了九章的答案才知道原来是用set来做。

public int longestPalindrome(String s) {
    if (s == null || s.isEmpty()) {
        return 0;
    }

    Set<Character> remaining = new HashSet<>();
    for (Character c : s.toCharArray()) {
        if (remaining.contains(c)) {
            remaining.remove(c);
        } else {
            remaining.add(c);
        }
    }

    int numOfCharsNotAbleToFormParlindrome = remaining.size();
    if (numOfCharsNotAbleToFormParlindrome > 0) {
         // Because one of them can put in the middle and form parlindrome with other chars
        numOfCharsNotAbleToFormParlindrome--;
    }

    return s.length() - numOfCharsNotAbleToFormParlindrome;
}