Leetcode diary: 6. Zigzag Conversion

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

what I notice is that 
    1.) start at 0, 
    2.) whenever starting at 0, we put in one letter into each row
    3.) after that we are at currentRow = numRows-1;
    4.) next we are at numRows-2;
    5.) we then only put a letter for row == numRows-2, all else is ""
    6.) we then decrement currentRow and repeat #5 until currentRow == 0 
    7.) we repeat 1 to 6 until all letters are used.
    we can then loop through the matrix and get the concatenated string
    we don't need a 2d array, we can just use array of strings

var convert = function(s, numRows) {    
    let currentRow = 0; 
    let currentSIndex = 0;
    let upwardIndex = numRows-1;
    const matrix = new Array(numRows).fill("");

    while (currentSIndex < s.length) {
        while(currentRow < numRows && currentSIndex < s.length) {
            matrix[currentRow++] += s[currentSIndex++]
        }
        currentRow--                    //cause currentRow === numRows at this point;
        if(numRows >= 2) currentRow--;  //All start at numRows-2 except if numRows === 1

        while(currentRow > 0) {
            upwardIndex = numRows-1;
            while(upwardIndex >-1 && currentSIndex < s.length) {
                if(upwardIndex === currentRow) {
                    matrix[upwardIndex] += s[currentSIndex++];
                } 
                upwardIndex--
            }
            currentRow--;
        }
    }

    return matrix.join("")
};