6. ZigZag Conversion

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

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function(s, numRows) {
    var data = new Array(numRows);
    var up = false; // up or down? used in-between tops and bottoms
    
    for(var i = 0, j = 0; i < s.length; i ++){
        // init arr elems
        if(data[j] === undefined){
            data[j] = "";
        }
        // append cur char
        data[j] += s.charAt(i);
        
        
        if(j == 0){
            // On top
            up = false; // Change direction
            j ++;
        } else if(j == numRows-1){
            // On bottom
            up = true;  // Then go up
            j --;
        } // in-between, follow prev direction
        else if(up){
            j --;
        } else {
            j ++;
        }
    }
    // Join/concat arr of str to single arr 
    return data.join("");
};

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