CODE 124. ZigZag Conversion

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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".

public String convert(String s, int nRows) {
	// Start typing your Java solution below
	// DO NOT write main() function
	if(nRows == 1){
		return s;
	}
	ArrayList<ArrayList<Character>> charLists = new ArrayList<ArrayList<Character>>();
	for (int i = 0; i < nRows; i++) {
		charLists.add(new ArrayList<Character>());
	}
	char[] cs = s.toCharArray();
	for (int i = 0; i < s.length(); i++) {
		if (i % (nRows - 1) == 0) {
			int j = 0;
			for (j = i; j < cs.length && j < i + nRows; j++) {
				charLists.get(j - i).add(cs[j]);
			}
			i = j - 1;
		} else {
			int row = charLists.size() - 1 - i % (nRows - 1);
			charLists.get(row).add(cs[i]);
		}
	}
	StringBuilder sb = new StringBuilder();
	for (ArrayList<Character> charList : charLists) {
		for (char c : charList) {
			sb.append(c);
		}
	}
	return sb.toString();
}