I believe this code is faster , u can easily aviod extra loop of 26 elements

char* pangrams(char* s) { int i =0; int table[52] = {0};

int alphabet_cnt = 0;

while(s[i] != '\0'){

    if(s[i]>=65 && s[i] <= 90){
        if(table[s[i] - 65] == 0 && table[s[i] - 65 + 26] == 0 ){
            alphabet_cnt++;
            table[s[i] - 65]++;
        }
    }
    else  if(s[i]>=97 && s[i] <= 122 ){
        if(table[s[i] - 71] == 0 && table[s[i] - 71 - 26] == 0){
            alphabet_cnt++;
            table[s[i] - 71]++;
        }

    }
    i++;
}

if(alphabet_cnt == 26)
    return "pangram";

return "not pangram";

}