0006: Zigzag Conversion

Problem Statement

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

Example 3:

Input: s = "A", numRows = 1
Output: "A"

Constraints:

  • 1 <= s.length <= 1000
  • s consists of English letters (lower-case and upper-case), ',' and '.'.
  • 1 <= numRows <= 1000

Code Solution

class Solution:
    def convert(self, s, numRows):
        if numRows == 1: return s
        out = [[] for ix in range(numRows)]
        px = 0
        reverse = True
        for cx, c in enumerate(s):
            if px == 0 or px == numRows - 1:
                reverse = not reverse
            out[px].append(c)
            px = px + (1 if not reverse else -1)
        
        return "".join(["".join(o) for o in out])