A way to reorder a string in a specific manner

Reorder-String

//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"

//The way I understand it : convert("ABCDEFGHIJKLMNOPQRST", 4) will transform the string into:
//A     G     M     S
//B  E  H  K  N  Q  T
//C  F  I  L  O  R
//D     J     P
//
//and it will return: "AGMBEHKNCFILDJ"





import Foundation

func convert(_ s: String, _ numRows: Int) -> String {
    var mainArray = [[String]]()
    
    for _ in 1...numRows{
        let singleArray = [String]()
        mainArray.append(singleArray)
    }
    
    
    let theString = Array(s.characters)
    var count = numRows
    var arrayCounter = 0
    
    if numRows <= 2 {
        for char in theString{
            if arrayCounter%numRows != 0{
                mainArray[numRows - 1].append("\(char)")
            }
            else{
                mainArray[0].append("\(char)")
            }
            arrayCounter += 1
        }
        var newString = ""
        for array in mainArray {
            newString += array.joined()
        }
        return newString
    }else {
        arrayCounter = 0
        var charCounter = 0
        while charCounter < theString.count  {
            if count == numRows || count == 2*numRows {
                arrayCounter = 0
            }
            if count == 2*numRows {
                //            mainArray[0].append(" ")
                count = 0
            }
            else if count == (numRows - 1) {
                //            mainArray[numRows - 1].append(" ")
            }
            else {
                mainArray[arrayCounter].append("\(theString[charCounter])")
                charCounter += 1
            }
            count += 1
            arrayCounter += 1
        }
        var newString = ""
        for array in mainArray {
            newString += array.joined()
        }
        return newString
    }
}

let string = "ABCDEFGHIJKLMN"

print(convert(string, 4))