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
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

var convert = function(s, numRows) {
    let res = [];
    let count = 0;
    let up = true;
    for(let i =0; i<s.length;i++){

        if(!res[count]) res[count] = []
        res[count].push(s[i])

        count = up ? count+1 : count-1

        if(count+1 == numRows) up = false;
        else if(count == 0) up = true;
    }
    let result = ''
    for(let i of res){
        result += i.join('')
    }
    return result;
};