Leetcode #6: Zigzag Conversion
In this guide, we solve Leetcode #6 Zigzag Conversion in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
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
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: String
Intuition
We need to scan characters while tracking positions or counts.
A simple state machine keeps the logic precise.
Approach
Iterate through the string once and update the state for each character.
Use a map or array if you need fast lookups.
Steps:
- Iterate through characters.
- Maintain necessary state.
- Build or validate the output.
Example
P A H N
A P L S I I G
Y I R
Python Solution
class Solution:
def convert(self, s: str, numRows: int) -> str:
if numRows == 1:
return s
g = [[] for _ in range(numRows)]
i, k = 0, -1
for c in s:
g[i].append(c)
if i == 0 or i == numRows - 1:
k = -k
i += k
return ''.join(chain(*g))
Complexity
The time complexity is and the space complexity is , where is the length of the string . The space complexity is , where is the length of the string .
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.