class Solution {
    public String convert(String s, int numRows) {
        StringBuilder[] lists = new StringBuilder[numRows];
        for (int i = 0; i < numRows; i++){
            lists[i] = new StringBuilder();
        }

        int n = s.length();
        int i = 0;
        while (i < n){
            for (int row = 0; row < numRows && i < n; row++){
                lists[row].append(s.charAt(i++));
            }
            for (int row = numRows - 2; row >= 1 && i < n; row--){
                lists[row].append(s.charAt(i++));
            }
        }

        StringBuilder ans = new StringBuilder();
        for (int k = 0; k < numRows; k++){
            ans.append(lists[k].toString());
        }

        return ans.toString();
    }
}