//Runtime: 5 ms, faster than 74.58% of Java online submissions for ZigZag Conversion.
//Memory Usage: 38.9 MB, less than 80.85% of Java online submissions for ZigZag Conversion.
class Solution {
  public String convert(String s, int numRows) {
    if(s == null || s.length() < 1 || numRows == 1 || numRows >= s.length()) return s;

    StringBuilder[] sbs = new StringBuilder[numRows];
    for(int i = 1; i < numRows; i++) sbs[i] = new StringBuilder();
    sbs[0] =new StringBuilder(s.length());

    char[] str = s.toCharArray();
    int indx = 0;
    int step = 1;
    for(int i = 0; i < str.length; i++){
        sbs[indx].append(str[i]);
        indx = indx + step;
        if(indx == numRows -1) step = -1;
        else if(indx == 0) step = 1;
    }
    for(int i = 1; i < numRows; i++)sbs[0].append(sbs[i].toString());
    return sbs[0].toString();
  }
}