[原始] 上海一战720

1. ABC-CBA=100A+10B+C-100C-10B-A=99(A-C), 99 can not divided by 7, therefore it has to be A-C divisible by 7. Therefore case 1: C=1, A=8, B=0-9 (Forgot whether B can be 0 or not), case 2: C=2, A=9, B=0-9, case 3: A=C, A=C=1, B=0-9, same for A=C=2, A=C=3 ... etc. Therefore it should be at least more than 100.
哦，可能我错了，不应该考虑A=C的情况，所以如果是那样的话就只有20个。

2. N is perfect square, therefore let's assume N=m2, therefore (N^(1/2)+1)^2=N+2N^(1/2)+1>=N, but it is also perfect square because of N=m2, (N^(1/2)+1)^2=(m+1)2