zigzag-conversion.rb

# 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

def convert(s, num_rows)
  return s if num_rows <= 1

  rows = Array.new(num_rows) {[]}
  going_down = true
  current_row = 0
  next_row_modifier = 1

  s.each_char do |c|
      rows[current_row] << c
      going_down = false if current_row == num_rows - 1
      going_down = true if current_row == 0
      next_row_modifier = going_down ?  1 : -1
      current_row += next_row_modifier
  end

  rows.flatten.join
end