输出单词中重复的字母，怎么用26个元素的数组对应26个字母

实在没有思路

 int letter[26];
char s[100];
for (int i = 0; i < 26; i++) letter[i] = 0;
scanf("%s", &s);
for (int i = 0; i < strlen(s); i++)
if (s[i] >= 'A' && s[i] <= 'Z')
letter[s[i] - 'A']++;
if (s[i] >= 'a' && s[i] <= 'z')
letter[s[i] - 'a']++;