1.计算1-2+3-4+5-6+.+1999-2000+2001
=1+3-2+5-5+7-6+...+2001-2000
=1*1001
=1001
2.计算1000+999-998-997+996+995-994-993+.+104+103-102-101
=1000-998+999-997+...+104-102+103-101
=225*2
=450