987654321×123456789-987654320×123456788
=987654321×（123456788+1）-987654320×123456788
=987654321×123456788+987654321-987654320×123456788
=（987654320+1-987654320）×123456788+987654321
=123456788+987654321
=1111111109
故答案为：1111111109．