[leetcode] 266. Palindrome Permutation 解题报告

题目链接：https://leetcode.com/problems/palindrome-permutation/

Given a string, determine if a permutation of the string could form a palindrome.

For example,
"code" -> False, "aab" -> True, "carerac" -> True.

Hint:

1. Consider the palindromes of odd vs even length. What difference do you notice?
2. Count the frequency of each character.
3. If each character occurs even number of times, then it must be a palindrome. How about character which occurs odd number of times?

思路：提示给了很明显的答案，如果所有的字符都出现偶数次，那么肯定可以回文．如果只有一个字符出现奇数次，肯定也可以回文．因此我们只要用一个hash表来记录每个字符的个数，最后再统计一下奇数字符的数目即可．

代码如下：

class Solution {public:    bool canPermutePalindrome(string s) {        unordered_map<char, int> hash;        for(auto ch: s) hash[ch]++;        int num = 0;        for(auto val: hash)            if(val.second % 2) num++;        return num <= 1;    }};