```
class Solution(object):
def convert(self, s, numRows):
"""
:type s: str
:type numRows: int
:rtype: str
"""
idx = 0
res = ['' for _ in xrange(numRows)]
group = 2 * numRows - 2
while idx<=len(s)-1:
for i in xrange(0, numRows):
if idx>=len(s): break
res[i] += s[idx]
idx += 1
start = -2
for i in xrange(numRows, group):
if idx>=len(s): break
res[start] += s[idx]
idx += 1
start -= 1
final = ''
for i in xrange(len(res)):
final += res[i]
return final
```