6-zig-zag-conversion.dart

class Solution {
  String convert(String s, int numRows) {
    if (numRows == 1 || s.length <= numRows) return s;

    List<StringBuffer> rows = List.generate(numRows, (_) => StringBuffer());

    int currentRow = 0;
    bool goingDown = false;

    for (int i = 0; i < s.length; i++) {
      rows[currentRow].write(s[i]);

      if (currentRow == 0 || currentRow == numRows - 1) {
        goingDown = !goingDown;
      }

      currentRow += goingDown ? 1 : -1;
    }

    return rows.map((row) => row.toString()).join();
  }
}

// Example Usage
void main() {
  Solution solution = new Solution();

  print(solution.convert("PAYPALISHIRING", 3)); // Output: "PAHNAPLSIIGYIR"
  print(solution.convert("PAYPALISHIRING", 4)); // Output: "PINALSIGYAHRPI"
  print(solution.convert("A", 1)); // Output: "A"
}