Huffman Coding Problem | Counting no of each type of alphabets(case insensitive) in a given string.

huffman.c

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

int main()
{
    char str[20];
    scanf("%[^\n]s",str);
    int *counter= (int*)calloc(26,sizeof(int));
    // int counter[26]={0};
    int pointer=0;
    do
    {
        if(str[pointer]>=97 && str[pointer]<=122)
        {
            int idx=(int)str[pointer]-97;
            counter[idx]++;
        }
        else if(str[pointer]>=65 && str[pointer]<=90)
        {
            int idx=(int)str[pointer]-65;
            counter[idx]++;
        }
        pointer++;
    }while(pointer<26);
    for(int i=0;i<26;i++)
    {
        if(counter[i]>0)
        {
            printf("%c : %d\n",((char)(65+i)),counter[i]);
        }
    }
    return 0;
}