LeetCode 267 - Palindrome Permutation II

题目描述：

Given a string s, return all the palindromic permutations (without duplicates) of it. Return an empty list if no palindromic permutation could be form.

Example 1:

Input: "aabb"

Output: ["abba", "baab"]

Example 2:

Input: "abc"

Output: []

class Solution {
public:
    vector generatePalindromes(string s) {
        unordered_map hash;
        for(auto c:s) hash[c]++;
        vector result;
        char center='\0';
        for(auto x:hash) // 确定奇数个的字符作为中心
        {
            if(x.second%2==1)
            {
                if(center!='\0') return result;
                else center=x.first;
            }
        }
        string cur;
        if(center!='\0') 
        {
            hash[center]--;
            if(hash[center]==0) hash.erase(center);
            cur.push_back(center);
        }
        permutate(cur,hash,result); // 将字符进行排列，两个相同字符可以放置于中心两端
        return result;
    }
    
    void permutate(string cur, unordered_map hash, vector& result)
    {
        if(hash.size()==0)
        {
            result.push_back(cur);
            return;
        }
        for(auto x:hash)
        {
            string next=x.first+cur+x.first;
            unordered_map temp=hash;
            temp[x.first]-=2;
            if(temp[x.first]==0) temp.erase(x.first);
            permutate(next,temp,result);
        }
    }
};