Zigzag Conversion

Problem Description

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 (JavaScript)

/**
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function(s, numRows) {
    if(numRows == 1) return s;
    
    var getChar = function(str, idx){
        if(idx < 0 || idx >= str.length) return '';
        return str.charAt(idx);
    }
    
    var out = '';
    var len = s.length;
    for(var i = 0; i < numRows; i++){
        var pos = 0;
        while(pos + 2 - numRows <= len){
            if(i == 0 || i == numRows - 1){
                out += getChar(s, pos + i);
            } else {
                out += getChar(s, pos - i) + getChar(s, pos + i);
            }
            pos += 2 * numRows - 2;
        }
    }
    
    return out;
};