409. Longest Palindrome

题目

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.

题目大意

给定一个包含大写字母和小写字母的字符串，找到通过这些字母构造成的最长的回文串。在构造过程中，请注意区分大小写。比如 "Aa" 不能当做一个回文字符串。注意:假设字符串的长度不会超过 1010。

解题思路

• 给出一个字符串，要求用这个字符串里面的字符组成一个回文串，问回文串最长可以组合成多长的？
• 这也是一题水题，先统计每个字符的频次，然后每个字符能取 2 个的取 2 个，不足 2 个的并且当前构造中的回文串是偶数的情况下(即每 2 个都配对了)，可以取 1 个。最后组合出来的就是最长回文串。

代码

package leetcode

func longestPalindrome(s string) int {
	counter := make(map[rune]int)
	for _, r := range s {
		counter[r]++
	}
	answer := 0
	for _, v := range counter {
		answer += v / 2 * 2
		if answer%2 == 0 && v%2 == 1 {
			answer++
		}
	}
	return answer
}