[leetcode] 6. zigzag conversion

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 text, int nRows);

convert(“PAYPALISHIRING”, 3) should return “PAHNAPLSIIGYIR”.

If nRows=4, text=abcdefghijklmnop

 
row 1:a   g   m
row 2:b f h l n
row 3:c e i k o
row 4:d   j   p      return row 1 + row 2 + row 3 +row 4 ,i.e. return agm bfhln ceiko djp

so,assigned order is：row 1, row 2, row 3, row 4, row 3, row 2, row 1,row 2, row 3 etc.
the ith char in text will assigned to the row which NO. is the value of the ith index of the above list. 

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        if numRows==1:
            return s

        li=['']*numRows
        po=-1
        dir=1
        for i in s:
            if dir:
                po+=1
                li[po]=li[po]+i
                if po==numRows-1:
                    dir=0
            else:
                po-=1
                li[po]=li[po]+i
                if po==0:
                    dir=1
        return ''.join(li)           

Status: Accepted
Runtime: 112 ms
Ranking: beats 92.15%