C/leetcode/src/917.c

char * reverseOnlyLetters(char * S){
    int last = strlen(S) - 1, i;
    for(i = 0; i < last;) {
        if(!isalpha(S[i])) {
            i++;
            continue;
        }
        if(!isalpha(S[last])) {
            last--;
            continue;
        }
        char tmp = S[i];
        S[i] = S[last];
        S[last] = tmp;
        i++;
        last--;
    }
    return S;
}