[leetcode] reorganize String

bool compare(pair<int,char>a, pair<int,char>b) {
    return a.first>b.first;
}

class Solution {
public:
    string reorganizeString(string S) {
        auto length=S.size();
        if (length<=1)
            return S;
        int counts[26]={0};
        for (int i=0;i<length;i++)
            ++counts[S[i]-‘a‘];
        int threshold=length/2+(length%2==0?0:1);
        for (int i=0;i<26;i++) {
            if (counts[i]>threshold)
                return "";
        }
        vector<pair<int,char>> characters_involved;
        for (int i=0;i<26;i++) {
            if (counts[i]>0) {
                characters_involved.push_back(make_pair(counts[i],‘a‘+i));
            }
        }
        sort(characters_involved.begin(), characters_involved.end(),compare);
        //sort characters_involved in decreasing order based on the counts.
        string answer(length, ‘ ‘);
        auto current_choice=characters_involved.begin();
        int current_position=0;
        while (current_choice!=characters_involved.end()) {
            answer[current_position]=current_choice->second;
            if (--(current_choice->first)==0)
                ++current_choice;
            current_position+=2;
            if (current_position>=length)
                current_position=1;
        }
        return answer;
    }
};