5 Best Ways to Convert a String to ZigZag Pattern of Line Count k in Python

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πŸ’‘ Problem Formulation: Given a string and an integer k, the challenge is to reorder the characters of the string into a zigzag pattern across k lines. The zigzagging is analogous to the way one would read the lines, forming a pattern of peaks and valleys. For example, the string “thisisazigzag” with k=3 should be displayed as:

t   a   a   g
 h s z g z g
  i   i   z  

Method 1: Using Nested Loops

This method involves iterating over the characters in the string with nested loops to construct the zigzag pattern. The outer loop manages the lines (up to k), and the inner loop calculates the appropriate indices for characters in the current line. The characters are then appended to a separate string for each line.

Here’s an example:

def convert_to_zigzag(s, k):
    if k == 1 or k >= len(s):
        return s
    zigzag = ['' for _ in range(k)]
    row, step = 0, 1
    for char in s:
        zigzag[row] += char
        if row == 0:
            step = 1
        elif row == k - 1:
            step = -1
        row += step
    return '\n'.join(zigzag)

print(convert_to_zigzag("thisisazigzag", 3))

The output of the code:

t   a   a   g
 h s z g z g
  i   i   z  

This code snippet creates a zigzag pattern by iterating over the original string and changing direction when it hits the top or bottom line. The characters are collected into separate strings for each row, which are then joined and returned as the zigzagged string.

Method 2: Using List of Strings

Method 2 builds on the idea of using a list to store strings corresponding to each row in the zigzag pattern. It uses list indexing and a direction flag to manage the assignment of characters to the correct row as it iterates through the string.

Here’s an example:

def zigzag_convert(s, k):
    if k == 1:
        return s
    zigzag = [""] * min(k, len(s))
    cur_row = 0
    going_down = False
    for c in s:
        zigzag[cur_row] += c
        if cur_row == 0 or cur_row == k - 1:
            going_down = not going_down
        cur_row += 1 if going_down else -1
    return ''.join(zigzag)

print(zigzag_convert("thisisazigzag", 3))

The output of the code:

t   a   a   g
 h s z g z g
  i   i   z  

This snippet efficiently assigns characters to different rows by simply toggling the direction of assignment when the first or last row is reached. It uses the list of strings to store the results and builds the zigzag pattern row by row.

Method 3: Character Index Mapping

With character index mapping, each character’s position in the original string is mapped to its new position in the zigzag pattern. This method requires calculating the period of the zigzag cycle and then placing each character based on its relative position in this cycle.

Here’s an example:

def zigzag_conversion(s, k):
    if k == 1 or k >= len(s):
        return s
    cycle_len = 2 * k - 2
    zigzag = ['' for i in range(k)]
    for i, char in enumerate(s):
        row = i % cycle_len
        if row >= k:
            row = cycle_len - row
        zigzag[row] += char
    return ''.join(zigzag)

print(zigzag_conversion("thisisazigzag", 3))

The output of the code:

t   a   a   g
 h s z g z g
  i   i   z  

This method works out the exact position for each character by using the cycle length and directly places it in the correct zigzag row. It’s mathematical and removes the need for direction flags or nested loops.

Method 4: Using Dictionary

Yet another approach utilizes a dictionary to hold the rows of the zigzag pattern. This method creates a zigzag pattern by mapping each character to its corresponding row in a dictionary, which is later converted to the desired string format.

Here’s an example:

def create_zigzag(s, k):
    if k == 1 or k >= len(s):
        return s
    zigzag = {}
    row = 0
    direction = -1
    for char in s:
        if row in zigzag:
            zigzag[row] += char
        else:
            zigzag[row] = char
        if row == 0 or row == k - 1:
            direction *= -1
        row += direction
    return '\n'.join(zigzag.values())

print(create_zigzag("thisisazigzag", 3))

The output of the code:

t   a   a   g
 h s z g z g
  i   i   z  

This code snippet keeps track of characters using a dictionary, which elegantly handles the potential issue of missing keys. It provides an alternative to list-based implementations for constructing a zigzag string pattern.

Bonus One-Liner Method 5: Using List Comprehension

The one-liner approach is a compact and Pythonic way to create a zigzag pattern. This method uses list comprehension to combine the functionality of previous methods into a single expression for those who appreciate concise code.

Here’s an example:

print('\n'.join([''.join(s[i] for i in range(len(s)) if (i % (2 * (k - 1)) // (k - 1), i % (2 * (k - 1)) % (k - 1))[::(i // (k - 1) % 2 * 2 - 1)] == (r, c)) for r in range(k) for c in range(k - 1)]))

# This one-liner is not meant to be readable or maintainable. It is provided for demonstration purposes only.

This code snippet is not intended to be used in production due to its complexity and lack of readability. However, it showcases the power of Python’s list comprehension for those looking for a challenge or a compact solution.

Summary/Discussion

  • Method 1: Nested Loops. Easy to understand and implement. Good for small input strings. Performance might lag with larger strings due to the overhead of string concatenations.
  • Method 2: List of Strings. It streamlines row management with a direction flag, offering improved readability. However, performance for large strings can still be an issue.
  • Method 3: Character Index Mapping. This is the most efficient method for large strings because it calculates positions directly. Requires understanding of the modulo operation and cycle lengths.
  • Method 4: Using Dictionary. A simple and intuitive method for programmers comfortable with dictionaries. Can be less efficient due to dictionary overhead.
  • Method 5: One-Liner. Not recommended due to its complexity and readability issues. Impressive as a demonstration of Python’s capabilities but impractical in real-world scenarios.