[LeetCode] 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" .

比较无聊的实现题，一个一个的处理zigzag。根据规律，对于每个zigzag先从上往下走一列（“|"），然后再从下往上走一个反对角线("/")。

	// convert at most 2 * nRows - 2 characters
	private int convert(String s, int start, int nRows,
			ArrayList<ArrayList<Character>> ret) {
		// process a column
		for (int i = 0; i < nRows; i++) {
			if (start + i < s.length()) {
				ret.get(i).add(s.charAt(start + i));
			} else {
				return s.length();
			}
		}

		start += nRows;

		// process a diagonal
		for (int i = 0; i < nRows - 2; i++) {
			if (start + i < s.length()) {
				ret.get(nRows - i - 2).add(s.charAt(start + i));
			} else {
				return s.length();
			}
		}

		return start + nRows - 2;
	}

	public String convert(String s, int nRows) {
		if (nRows == 1)
			return s;

		ArrayList<ArrayList<Character>> ret = new ArrayList<ArrayList<Character>>();
		for (int i = 0; i < nRows; i++) {
			ret.add(new ArrayList<Character>());
		}

		int start = 0;

		while (start < s.length()) {
			start = convert(s, start, nRows, ret);
		}

		// convert the result into string
		StringBuffer stringBuffer = new StringBuffer();
		for (ArrayList<Character> list : ret) {
			for (char c : list) {
				stringBuffer.append(c);
			}
		}

		return stringBuffer.toString();
	}