6. ZigZag Conversion

class Solution {
    public String convert(String s, int numRows) {

        int len = s.length();
        if( len < 2 || numRows == 1)
            return s;

        StringBuffer[] sb = new StringBuffer[numRows];
        for (int i = 0; i < numRows; i++) {     // 初始化
            sb[i] = new StringBuffer();
        }

        int row = 1, index = 0;
        boolean flag = true;
        while(index < len) {

            sb[row - 1].append(s.charAt(index));
            if(flag == true) {      //下降
                row++;
                if(row == numRows)
                    flag = false;
            }
            else {                  //上升
                row--;
                if(row == 1)
                    flag = true;
            }
            index++;
        }

        for (int i = 1; i < numRows; i++) {
            sb[0].append(sb[i].toString());
        }

        return sb[0].toString();
    }
}