zigzag-conversion.js

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */

var convert = function(s, numRows) {
   
    if (numRows == 1) {
        return s;
    }

    // Start the process on row 1, heading down
    let currentRow = 1;
    let headingDown = true;

    // Initialise an array to store the zigzag data
    let zigZagArray = [];

    // Loop through the requested number of rows
    for (let i = 0; i < numRows; i++) {
        // Initialise each zigzag row as an empty array
        zigZagArray[i] = [];
    }

    // Loop through the string
    for (let i = 0; i < s.length; i++) {
        // Add the current letter to the current row
        zigZagArray[currentRow - 1].push(s[i]);

        if (headingDown) {
            currentRow++;

            // Check if we've exceeded the maximum number of rows
            if (currentRow > numRows) {
                // Start heading back up again
                currentRow = numRows - 1;
                headingDown = false;
            }
        } else {
            currentRow--;

            // Check if we've exceeded the top row
            if (currentRow < 1) {
                // Start heading down again
                currentRow = 2;
                headingDown = true;
            }
        }
    }

    // Initialise a return string
    let totalString = '';

    // Loop through the constructed rows
    for (let i = 0; i < numRows; i++) {
        // Append the row's characters joined together
        totalString += zigZagArray[i].join('');
    }

    return totalString;
    
};