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 text, int nRows); convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

难度：medium

Solution: Linear Function

class Solution {
    public String convert(String s, int numRows) {
        if(s.length() <= numRows || numRows == 1) {
            return s;
        }
        List<StringBuilder> tempRes = new ArrayList<>();
        for(int i = 0; i < numRows; ++i) {
            tempRes.add(new StringBuilder());
        }

        int direction = 1;
        for(int i = 0, j = 0; i < s.length(); ++i, j += direction) {
            tempRes.get(j).append(s.charAt(i));
            if(j == 0) {
                direction = 1;
            } else if(j == numRows - 1) {
                direction = -1;
            }
        }

        StringBuilder res = new StringBuilder();
        for(StringBuilder t : tempRes) {
            res.append(t);
        }
        return res.toString();
    }
}