Zigzag conversion

Time: O(N); Space: O(1); easy

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”

Notice that zigzag extends according to the directions of down->up-right->down->up-right…

|   /|   /|
|  / |  / | ...
| /  | /  | ...
|/   |/   |/

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

Example 1:

Input: s = “PAYPALISHIRING”, numRows = 3

Output: “PAHNAPLSIIGYIR”

Example 2:

Input: s = “PAYPALISHIRING”, numRows = 4

Output: “PINALSIGYAHRPI”

Explanation:

  • After conversion, we get

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

  • Read line by line, the answer is “PINALSIGYAHRPI”.

Example 3:

Input: s = “PAYPALISHIRING”, numRows = 1

Output: “PAYPALISHIRING”

Explanation:

  • After conversion, we get PAYPALISHIRING

  • Read line by line, the answer is “PAYPALISHIRING”.

[1]:

class Solution1(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        if numRows == 1:
            return s

        step, zigzag = 2 * numRows - 2, ""
        for i in range(numRows):
            for j in range(i, len(s), step):
                zigzag += s[j]
                if 0 < i < numRows - 1 and j + step - 2 * i < len(s):
                    zigzag += s[j + step - 2 * i]

        return zigzag

[2]:

sol = Solution1()

s = "PAYPALISHIRING"
numRows = 3
assert sol.convert(s, numRows) == "PAHNAPLSIIGYIR"

s = "PAYPALISHIRING"
numRows = 4
assert sol.convert(s, numRows) == "PINALSIGYAHRPI"

s = "PAYPALISHIRING"
numRows = 1
assert sol.convert(s, numRows) == "PAYPALISHIRING"