示例1：
Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
示例2：
Python解决方案：
def convert(self, s, numRows):
"""
:type s: str
:type numRows: int
:rtype: str
"""
if len(s) <= numRows or numRows==1 :
return s
s_dict = {i:"" for i in range(numRows)}
unit = 2*numRows - 2
for i in range(len(s)):
remain = i%unit
if remain <= numRows - 1:
s_dict[remain] += s[i]
else:
pos = unit - remain
s_dict[pos] += s[i]
out = ""
for i in range(numRows):
out += s_dict[i]
return out