LeetCode - 409. Longest Palindrome

最新推荐文章于 2018-07-18 14:27:46 发布

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本文介绍了一种算法，用于从给定字符串中构建最长的回文串。通过统计字符出现次数并考虑回文串的特点，该算法能高效地计算出可构建的最长回文串长度。

问题

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.

代码

class Solution {
public:
    int longestPalindrome(string s) {
        vector<int> v(58,0);    // A-z 65-122，因此开辟58个空间即可
        int flag = 0, length = 0;
        for(int i=0; i<s.size(); i++)
            v[s[i]-65]++;

        for(int j=0; j<58; j++)
        {
            if(v[j] == 1) flag = 1;
            else if(v[j])
            {
                if(v[j]%2 == 0) length += v[j];
                else 
                {
                    length += (v[j]-1);
                    flag = 1;
                }
            }
        }
        if(flag) length++;
        return length;
    }

    //参考答案，贵在精简，不过时间复杂度一样O（n^2），空间花费多一些
    int longestPalindrome(string s) {
        vector<int> m(256, 0);        
        for (auto& c : s) m[c-'\0']++;
        int result = 0;
        for (auto& i : m) result += i%2 ? (result%2 ? i-1 : i) : i; //result%2判断是否已经采用过奇数个数的字母
        return result;
    }
};