find the nth digit

find the nth digit 假设： S1 = 1 S2 = 12 S3 = 123 S4 = 1234 ......... S9 = 123456789 S10 = 1234567891 S11 = 12345678912 ............ S18 = 123456789123456789 .................. 现在我们把所有的串连接起来 S = 11212312

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