zigzag_conversion.dart

/*


-* 6. Zigzag Conversion *-

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R
And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);
 

Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:
P     I    N
A   L S  I G
Y A   H R
P     I
Example 3:

Input: s = "A", numRows = 1
Output: "A"
 

Constraints:

1 <= s.length <= 1000
s consists of English letters (lower-case and upper-case), ',' and '.'.
1 <= numRows <= 1000

*/
class A {
  String convert(String s, int numRows) {
    if (numRows == 1) return s;
    List<String> rows = List.filled(numRows, "");
    int j = 0, d = 1;
    for (int i = 0; i < s.length; i++) {
      rows[j] += s[i];
      if (j == numRows - 1)
        d = -1;
      else if (j == 0) d = 1;
      j += d;
    }

    String res = "";
    for (int i = 0; i < numRows; i++) {
      res += rows[i];
    }

    return res;
  }
}