ZigZag

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R
And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);
Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I


My code:
import numpy as np
class Solution:
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        length=len(s)
        if length<=numRows:
            return s
        if numRows==1:
            return s
        batch=numRows*2-2
        times=int(np.ceil(length/batch))
        s1=""
        for j in range(numRows):
            for i in range(times):
            
                if j==0:
                    index=i*batch+j
                    if index<len(s):
                        s1=s1+s[index]
                elif j==numRows-1:
                    
                    index=(i+1)*batch-(numRows-1)
                    #print(index)
                    if index<len(s):
                        s1=s1+s[index]
                else:
                   
                    index1=i*batch+j
                    #print(index1)
                    if index1<len(s):
                        s1=s1+s[index1]
                    index2=(i+1)*batch-(j)
                    #print(index2)
                    if index2<len(s):
                        s1=s1+s[index2]                    
       
        return s1
  Solution:
  https://leetcode.com/problems/zigzag-conversion/