Hashing-maximum occurring character in string

/*
Write an efficient function to return maximum occurring character in the input string e.g., if input
string is “test” then function should return ‘t’.
note: use hashing */

#include<bits/stdc++.h>
using namespace std;

int main()
{
    string s;
    cout<<"Enter the String::";
    cin>>s;
    int a[26]={0};
    
    // H(x)=(ascii(charater)-19)%26  <-Hash function
    //hence index 0,1,2,3,... ,25   represents a,b,c, ... ,z respectively.
    
    for(int i=0;i<s.size();i++)
    {
        int h=(int(s[i])-19)%26;
        a[h]++;
    }
    int index=0;
    for(int i=1;i<26;i++)
    {
        if(a[i]>a[index])
        {
            index=i;
        }
    }
    //character=char(index+97)
    cout<<"\nMaximum occuring character:: "<<char(97+index); 
}