Stack 1,10,100,1000,10000,100000,... vertically:

1
10
100
1000
10000
100000
...
the cumulative sums of digits from the top are:
1,3,6,10,15,21,... which are in the form of combination(N,2) = N*(N-1)/2

checking whether one is at Kth digit is the same as finding if there is integer solution to:
N*(N-1)/2 = K-1

one way is to solve the quadratic equation for N (in double) and cast N to integer and check the above relation.
regards,
So Sui Ming



Edited by author 16.01.2024 19:27

Edited by author 16.01.2024 19:27