6. Zigzag Conversion Leetcode Solution in JavaScript (JS)

Problem

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

Brute force with O(n) complexity

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function(s, numRows) {
    let inputString = s;
    let targetRows = numRows;
    if (targetRows == 1) {
        return inputString;
    }

    // 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 < targetRows; i++) {
        // Initialise each zigzag row as an empty array
        zigZagArray[i] = [];
    }

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

        if (headingDown) {
            currentRow++;

            // Check if we've exceeded the maximum number of rows
            if (currentRow > targetRows) {
                // Start heading back up again
                currentRow = targetRows - 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 < targetRows; i++) {
        // Append the row's characters joined together
        totalString += zigZagArray[i].join('');
    }

    return totalString;
};