How do you solve Decode the Slanted Ciphertext on LeetCode?

Decode the Slanted Ciphertext (LeetCode #2075) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Understanding how to fill a matrix in a zigzag pattern is crucial for solving this problem.

What is the brute force solution for Decode the Slanted Ciphertext?

The brute force approach for Decode the Slanted Ciphertext is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute-force approach involves reconstructing the matrix from the encoded text based on the given number of rows. We fill the matrix in a zigzag manner and then read it row-wise to retrieve the original text. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Decode the Slanted Ciphertext?

Decode the Slanted Ciphertext uses the following concepts: String, Simulation. The recommended patterns to study are: Matrix Manipulation, Zigzag Traversal.

What are the interview tips for Decode the Slanted Ciphertext?

Always clarify the input and output requirements before diving into coding. Think about edge cases, such as when the encoded text is shorter than the number of rows. Practice filling matrices in different patterns, as this is a common theme in many problems.

What are common mistakes when solving Decode the Slanted Ciphertext?

Not correctly calculating the number of columns based on the number of rows. Failing to handle empty spaces correctly when reconstructing the original text.

#2075

Decode the Slanted Ciphertext

Medium

StringSimulationMatrix ManipulationZigzag Traversal

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

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Intuition

Time O(n)Space O(n)

The optimal solution builds the matrix directly from the encoded text and reconstructs the original text in a single pass, avoiding unnecessary iterations. This approach is efficient and straightforward.

⚙️

Algorithm

4 steps

1Step 1: Calculate the number of columns needed using the length of encodedText and the number of rows.
2Step 2: Create a 2D matrix to hold the characters.
3Step 3: Fill the matrix in a zigzag pattern using a single loop.
4Step 4: Read the matrix row-wise to reconstruct the original text.

solution.py21 lines

1# Full working Python code
2
3def decodeSlantedCiphertext(encodedText: str, rows: int) -> str:
4    n = len(encodedText)
5    cols = (n + rows - 1) // rows
6    matrix = [[' ' for _ in range(cols)] for _ in range(rows)]
7    index = 0
8
9    # Fill the matrix in a zigzag manner
10    for diag in range(rows + cols - 1):
11        for r in range(rows):
12            c = diag - r
13            if c >= 0 and c < cols:
14                if index < n:
15                    matrix[r][c] = encodedText[index]
16                    index += 1
17
18    # Read the matrix row-wise to form the original text
19    originalText = ''.join(''.join(row) for row in matrix).rstrip()
20    return originalText
21

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Complexity note: The time complexity is O(n) because we traverse the encoded text once to fill the matrix and then read it. The space complexity is O(n) due to the storage of the matrix.

1Understanding how to fill a matrix in a zigzag pattern is crucial for solving this problem.
2Recognizing the relationship between the number of rows, columns, and the length of the encoded text helps in efficient reconstruction.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.