Maximum Length of a Concatenated String with Unique Characters

 https://leetcode.com/problems/maximum-length-of-a-concatenated-string-with-unique-characters/

class Solution {
public:
    bool checkUnique(string s){
        int count[26]={0};
        for(int i=0;i<s.size();i++){
            if(count[s[i]-'a']==1)
                return false;
            else
                count[s[i]-'a']++;
        }
        return true;
    }
    
    void backtrack(vector<string>& arr,string s,int index,int &count){
        if(checkUnique(s))
            count=(s.size()>count)?s.size():count;
        
        for(int i=index;i<arr.size();i++)
            backtrack(arr,s+arr[i],i+1,count);
    }
    
    int maxLength(vector<string>& arr) {
       int count=0;
        if(arr.size()<0) return 0;
        if(arr.size()==1)
            return arr[0].size();
        backtrack(arr,"",0,count);
        return count;
    }
};