findChar.c

#include <assert.h>

char find(const char *str) {
    int cnt = 1;
    char c = *str++;
    for ( ; *str; ++str) {
        c = cnt == 0 ? *str : c;
        cnt = cnt + (*str == c ? 1 : (cnt - 1 < 0 ? -cnt : -1));
    }
    return cnt > 0 ? c : 0;
}

int main(int argc, char* argv[]) {
    assert(find("abcd") == 0);
    assert(find("aba") == 'a');
    assert(find("abba") == 0);
    assert(find("ababa") == 'a');
    assert(find("ababb") == 'b');
  return 0;
}