409. Longest Palindrome(计算一组字符集合可以组成的回文字符串的最大长度)

Posted 2021-11-29 shaer

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.

解释：构成回文字符串，最好的情况就是所有出现双数的字母都排上，中间再加一个单着的

方法一：哈希表

class Solution {
    public int longestPalindrome(String s) {
        int count=0;
        int tip=0;
        HashMap<Character,Integer> map=new HashMap<Character,Integer>();
        for(char c:s.toCharArray()){
           if(map.containsKey(c)) {
               int nums=map.get(c);
               map.put(c,nums+1);
           }else{
               map.put(c,1);
           }
        }
        
        for(char c:s.toCharArray()){
            if(map.containsKey(c)) {
               int nums=map.get(c);
               if(nums%2==1){
                   tip=1;
               }
                count+=nums/2;
                map.remove(c);
           }
        }
        return count*2+tip;
    }
}

 

方法二：数组
这种方法思路很简单，但是空间和时间消耗都很大。

class Solution {
    public int longestPalindrome(String s) {
        int[] chr = new int[256];
        for(char c:s.toCharArray()){
            chr[c]++;
        }
        int p=0;
        for(int c:chr){
            p+=(c/2)*2;
        }
        if(p<s.length()){
            p++;
        }
        return p;
    }
}