答案都写错了。。。
易得 0<=a0<=2;  1<=a1<=2 ,2<=a2<=3,... 20<=a20<=21
所以：a1=2-|a0-1|,a2=a1+1= 3-|a0-1| ,...a20 =a19+1=20-|a0-1| ,
然后，若a0<=1,则sum=XXX    ;若a0>1,则sum=XXX
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FROM 120.244.162.*