409. Longest Palindrome | 判断能组成的最长回文

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本文介绍了一个算法问题：如何从给定字符串中构建最长的回文串。通过详细解析案例，展示了考虑字符出现次数来确定回文串长度的方法，并提供了一段实现该功能的Java代码。

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Given a string which consists of lowercase or uppercase letters, find the length of the longest palindromes that can be built with those letters.

This is case sensitive, for example "Aa" is not considered a palindrome here.

Note:
Assume the length of given string will not exceed 1,010.

Example:

Input:
"abccccdd"

Output:
7

Explanation:
One longest palindrome that can be built is "dccaccd", whose length is 7.

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编程中要注意是，如果有单个的最后要加上1，因为可以放在中间。

public class Solution {
    public int longestPalindrome(String s) {
		int len = 0;
		int[] a = new int[52];
		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) <= 'Z') {
				a[s.charAt(i) - 'A']++;
			} else {
				a[s.charAt(i) - 'a' + 26]++;
			}
		}

		for (int j = 0; j < a.length; j++) {
			if (a[j] != 0) {
				if (a[j] % 2 == 0) {
					len += a[j];
				} else {
					if (len % 2 == 0) {
						len += a[j];
					} else {
						len = len + a[j] - 1;
					}
				}
			}
		}
		return len;
	}
}