[jeetcode]ZigZag Conversion

这道题目属于比较简单的，但是仍然要注意空间复杂度，比较好的解法是运用数学手段，发现规律。

public class Solution {
    public String convert(String s, int nRows) {
        int len = s.length();
        if (len <= nRows || nRows == 1) {
            return s;
        } else {
            String res = "";
            int num = 0;
            for (int r = 0; r < nRows; r++) {
                int count = 0;
                num = r;
                while (num < len) {
                    res = res + s.charAt(num);
                    count = count + 1;
                    if (r == 0 || r == nRows - 1) {
                        num = num + 2 * (nRows - 1);
                    } else {
                        if (count % 2 == 1) {
                            num = num + 2 * (nRows - r - 1);
                        } else {
                            num = num + 2 * r;
                        }
                    }
                }
            }
            return res;
    }
    }
}