6. ZigZag Conversion

class Solution {
    public String convert(String s, int numRows) {
        StringBuilder[] sb = new StringBuilder[numRows];
        for (int i = 0; i < sb.length; i++) {
            sb[i] = new StringBuilder();
        }
        int i = 0;
        while (i < s.length()) {
            for (int j = 0; j < numRows && i < s.length(); j ++) {
                sb[j].append(s.charAt(i));
                i ++;
            }
            for (int k = numRows - 2; k > 0 && i < s.length(); k --) {
                sb[k].append(s.charAt(i));
                i ++;
            }
        }
        StringBuilder res = new StringBuilder();
        for (i = 0; i < sb.length; i++) {
            res.append(sb[i]);
        }
        return res.toString();
    }
}

public class Solution {
    public String convert(String s, int numRows) {
        // Write your code here
        if (numRows == 1) {
            return s;
        }
        StringBuilder[] sbList = new StringBuilder[numRows];
        for (int i = 0; i < sbList.length; i++) {
            sbList[i] = new StringBuilder();
        }
        for (int i = 0; i < s.length(); i++) {
            int mod = i % ((numRows - 1) * 2);
            if (mod < numRows) {
                sbList[mod].append(s.charAt(i));
            } else {
                sbList[(numRows - 1) * 2 - mod].append(s.charAt(i));
            }
        }
        StringBuilder res = new StringBuilder();
        for (StringBuilder sb : sbList) {
            res.append(sb);
        }
        return res.toString();
    }
}