2075. Decode the Slanted Ciphertext

String
Matrix
Simulation

TIME

O(n)

SPACE

O(n)

Intuition

The encoding places characters diagonally and reads row by row. To decode, reconstruct the matrix from row-wise data and read diagonally.

Solution

Step 1: Calculate Dimensions

cols = len(encodedText) / rows

Step 2: Reconstruct Matrix

The encodedText is the matrix read row by row, so:

Row i contains encodedText[i * cols : (i + 1) * cols]

Step 3: Read Diagonally

For each starting column c (0 to cols-1):

Read characters at positions (0, c), (1, c+1), (2, c+2), ...
Stop when row or column goes out of bounds

Step 4: Strip Trailing Spaces

The original text has no trailing spaces.

Visual Example

encodedText = "ch   ie   pr", rows = 3, cols = 4

Matrix:
  c=0 c=1 c=2 c=3
r=0: c   h   ' ' ' '
r=1: ' ' i   e   ' '
r=2: ' ' ' ' p   r

Diagonal reads:
Start c=0: (0,0)→c, (1,1)→i, (2,2)→p = "cip"
Start c=1: (0,1)→h, (1,2)→e, (2,3)→r = "her"
Start c=2: (0,2)→' ', (1,3)→' ' = "  "
Start c=3: (0,3)→' ' = " "

Combined: "cipher  " → strip → "cipher"

Complexity

Time: O(n) where n = len(encodedText)
Space: O(n) for result string (or O(1) if using index math directly)

Edge Cases

rows = 1 → return encodedText stripped
Empty string → return empty
All spaces except diagonal → still works

Code

class Solution:
    def decodeCiphertext(self, encodedText: str, rows: int) -> str:
        """
        2075. Decode the Slanted Ciphertext
        Time: O(n)
        Space: O(n)
        """
        if not encodedText:
            return ""

        cols = len(encodedText) // rows
        result = []

        for start_col in range(cols):
            row, col = 0, start_col
            while row < rows and col < cols:
                result.append(encodedText[row * cols + col])
                row += 1
                col += 1

        return ''.join(result).rstrip()

/**
 * 2075. Decode the Slanted Ciphertext
 * Time: O(n)
 * Space: O(n)
 */
class Solution {
    public String decodeCiphertext(String encodedText, int rows) {
        if (encodedText.isEmpty()) {
            return "";
        }

        int cols = encodedText.length() / rows;
        StringBuilder result = new StringBuilder();

        for (int startCol = 0; startCol < cols; startCol++) {
            int row = 0, col = startCol;
            while (row < rows && col < cols) {
                result.append(encodedText.charAt(row * cols + col));
                row++;
                col++;
            }
        }

        int end = result.length();
        while (end > 0 && result.charAt(end - 1) == ' ') {
            end--;
        }

        return result.substring(0, end);
    }
}