LeetCode in Python

6. Zigzag Conversion

Medium

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

Example 3:

Input: s = “A”, numRows = 1

Output: “A”

Constraints:

• 1 <= s.length <= 1000
• s consists of English letters (lower-case and upper-case), ',' and '.'.
• 1 <= numRows <= 1000

Here are the steps to solve the “Zigzag Conversion” problem:

Approach:

1. Handle Special Cases:
   • If the number of rows (numRows) is 1 or the length of the string (s) is less than or equal to the number of rows, return the original string.
2. Initialize Rows:
   • Create a list of strings to represent each row of the zigzag pattern.
   • Initialize an index variable to keep track of the current row.
3. Iterate through the String:
   • Iterate through each character in the string s.
4. Fill Rows:
   • For each character, determine the current row in the zigzag pattern.
   • Append the character to the corresponding row.
5. Read Zigzag Pattern:
   • Concatenate the rows to obtain the final zigzag pattern.
6. Return Result:
   • Return the final zigzag pattern.

Python Code:

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        # Handle special cases
        if numRows == 1 or len(s) <= numRows:
            return s

        # Initialize rows
        rows = [''] * numRows
        index = 0
        direction = 1  # 1 for down, -1 for up

        # Iterate through the string
        for char in s:
            # Fill rows
            rows[index] += char

            # Change direction if reaching top or bottom row
            if index == 0:
                direction = 1
            elif index == numRows - 1:
                direction = -1

            # Move to the next row
            index += direction

        # Read zigzag pattern
        result = ''.join(rows)

        # Return the final result
        return result

# Example Usage:
solution = Solution()

# Example 1:
s1 = "PAYPALISHIRING"
numRows1 = 3
print(solution.convert(s1, numRows1))  # Output: "PAHNAPLSIIGYIR"

# Example 2:
s2 = "PAYPALISHIRING"
numRows2 = 4
print(solution.convert(s2, numRows2))  # Output: "PINALSIGYAHRPI"

# Example 3:
s3 = "A"
numRows3 = 1
print(solution.convert(s3, numRows3))  # Output: "A"

This code defines a Solution class with a method convert that takes a string s and the number of rows numRows as input and returns the zigzag conversion of the string. The example usage demonstrates how to create an instance of the Solution class and call the convert method with different inputs.

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