s0006_zigzag_conversion

6. Zigzag Conversion

Medium

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

Solution

# @param {String} s
# @param {Integer} num_rows
# @return {String}
def convert(s, num_rows)
  s_len = s.length
  return s if num_rows == 1

  max_dist = num_rows * 2 - 2
  buf = ''

  (0...num_rows).each do |i|
    index = i

    if i.zero? || i == num_rows - 1
      while index < s_len
        buf << s[index]
        index += max_dist
      end
    else
      while index < s_len
        buf << s[index]
        index += max_dist - i * 2

        break if index >= s_len

        buf << s[index]
        index += i * 2
      end
    end
  end

  buf
end