string = "123456789" , numRows = 3
ans = "159246837"
explanation
    1   5   9
    2 4 6 8
    3   7

string = "HelloWorldKotlin" , k = 4
ans = "HoteWrollolKildn"
explanation
    H     o     t
    e   W r   o l
    l o   l K   i
    l     d     n

if string size is smaller than numRows or numRows is smaller than 2
then we can return string because it will make no changes to string.
If not than
we initiate three variable depth which is equalvalent to numRows ,
height which starts with 1 and start with starting index of string.
then we generate spaces to skip using formula 2 + (( n - 1 ) * 2 )
for both height and depth
with each iteration we decrement depth and increase height and start 
by one also we keep concatenating character on to new string with first
depth spaces and later height spaces that we generated using formula
if not zero