The problem can be treated as converting len(s) into （2nRows - 2）sequence.

```
class Solution:
# @return a string
def convert(self, s, nRows):
if nRows == 1:
return s
array = ['']*nRows
for i in range(len(s)):
index = i%(2*nRows-2)
if index < nRows:
array[index] += s[i]
else:
array[2*nRows - index- 2] += s[i]
out = ''
for i in array:
out = out + i
return out
```