6. Zigzag Conversion 🔀

Difficulty: Medium - Tags: String, Simulation

LeetCode Problem Link

Description

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this:

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);

Examples

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

• The input string s consists of printable ASCII characters.

• numRows is an integer in the range [1, 1000].

Solution 💡

To solve this problem, we simulate the zigzag pattern by appending characters to each row based on their current position. We traverse the string s and update the direction (up or down) when reaching the first or last row. After processing all characters, we concatenate the rows to get the final result.

Java

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) {
            return s;
        }

        StringBuilder[] rows = new StringBuilder[Math.min(numRows, s.length())];
        for (int i = 0; i < rows.length; i++) {
            rows[i] = new StringBuilder();
        }

        int currentRow = 0;
        boolean goingDown = false;

        for (char c : s.toCharArray()) {
            rows[currentRow].append(c);
            if (currentRow == 0 || currentRow == numRows - 1) {
                goingDown = !goingDown;
            }
            currentRow += goingDown ? 1 : -1;
        }

        StringBuilder result = new StringBuilder();
        for (StringBuilder row : rows) {
            result.append(row);
        }

        return result.toString();
    }
}

Time Complexity ⏳

• O(n): The time complexity is linear, where n is the length of the string s.

Space Complexity 💾

• O(n): The space complexity is also linear, as we are storing the zigzag pattern in a list of string builders.

You can find the full solution here.