C program to print Unique Letters in a string

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

int main()
{
    int arr[26] = {0};
    char string[100];
    int i;
    printf("Enter a sentence: \n");

    gets(string);
    for(i=0; i<strlen(string); i++)
    {

        arr[(tolower(string[i]))-97] = 1;

    }

    printf(" The Unique letters in the string are : \n");

    for(int i=0; i<26; i++)
    {
        if(arr[i] > 0)

            printf("%c  ", (i+97));
    }


    return 0;
}