6. ZigZag Conversion

Difficulty:
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    Problem

    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
    

    Solution

    /**
     * @param {string} s
     * @param {number} numRows
     * @return {string}
     */
    var convert = function(s, numRows) {
      if (s.length <= numRows || numRows < 2) return s;
      var len = s.length;
      var num = 2 * (numRows - 1);
      var res = Array(numRows).fill('');
      var tmp = 0;
      for (var i = 0; i < len; i++) {
        tmp = i % num;
        if (tmp < numRows) {
          res[tmp] += s[i];
        } else {
          res[num - tmp] += s[i];
        }
      }
      return res.join('');
    };
    

    Explain:

    曲线每 2 * (numRows - 1) 个数按规律出现。

    Complexity: