2075-decode-the-slanted-ciphertext.js

/**
 * 2075. Decode the Slanted Ciphertext
 * https://leetcode.com/problems/decode-the-slanted-ciphertext/
 * Difficulty: Medium
 *
 * A string originalText is encoded using a slanted transposition cipher to a string encodedText
 * with the help of a matrix having a fixed number of rows rows.
 *
 * originalText is placed first in a top-left to bottom-right manner.
 *
 * The blue cells are filled first, followed by the red cells, then the yellow cells, and so on,
 * until we reach the end of originalText. The arrow indicates the order in which the cells are
 * filled. All empty cells are filled with ' '. The number of columns is chosen such that the
 * rightmost column will not be empty after filling in originalText.
 *
 * encodedText is then formed by appending all characters of the matrix in a row-wise fashion.
 *
 * The characters in the blue cells are appended first to encodedText, then the red cells, and so
 * on, and finally the yellow cells. The arrow indicates the order in which the cells are accessed.
 *
 * For example, if originalText = "cipher" and rows = 3, then we encode it in the following manner.
 *
 * The blue arrows depict how originalText is placed in the matrix, and the red arrows denote the
 * order in which encodedText is formed. In the above example, encodedText = "ch ie pr".
 *
 * Given the encoded string encodedText and number of rows rows, return the original string
 * originalText.
 *
 * Note: originalText does not have any trailing spaces ' '. The test cases are generated such that
 * there is only one possible originalText.
 */

/**
 * @param {string} encodedText
 * @param {number} rows
 * @return {string}
 */
var decodeCiphertext = function(encodedText, rows) {
  if (rows === 1) return encodedText;

  const count = encodedText.length / rows;
  let result = '';

  for (let i = 0; i < count; i++) {
    for (let row = 0; row < rows && row + i < count; row++) {
      result += encodedText[row * count + i + row];
    }
  }

  return result.trimEnd();
};