串S=""(空串),子串数目只有1种:""
同样,当S="A",子串有"A"和""两个
当S="AB",子串有 "A" "B" "AB" ""
当S="ABC"子串有 "A" "B" "C" "AB" "AC" "BC" "ABC" ""
S="ABC" 其实就是 S[]={'A','B','C','\0'}