原始字符串是"def"， 打印得到下列所有组合情况：
                "d" "e" "f" "de" "df" "ed" "ef" "fd" "fe" "def" "dfe" "edf" "efd" "fde" "fed"

原始字符串是"def"， 打印得到下列所有组合情况：
                "d" "e" "f" "de" "df" "ed" "ef" "fd" "fe" "def" "dfe" "edf" "efd" "fde" "fed"