6. Zigzag Conversion

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

Approach 01: Simulate Zigzag Row by Row

class Solution {
public:
    string convert(string s, int numRows) {
        if (numRows == 1 || numRows >= s.length()) return s;
        vector rows(numRows);
        int currRow = 0;
        bool goingDown = false;
        for (char c : s) {
            rows[currRow] += c;
            if (currRow == 0 || currRow == numRows - 1) goingDown = !goingDown;
            currRow += goingDown ? 1 : -1;
        }
        string ret;
        for (string row : rows) ret += row;
        return ret;
    }
};

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows == 1 or numRows >= len(s):
            return s
        rows = [''] * numRows
        currRow = 0
        goingDown = False
        for c in s:
            rows[currRow] += c
            if currRow == 0 or currRow == numRows - 1:
                goingDown = not goingDown
            currRow += 1 if goingDown else -1
        return ''.join(rows)

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) return s;
        List rows = new ArrayList<>();
        for (int i = 0; i < Math.min(numRows, s.length()); i++)
            rows.add(new StringBuilder());
        int currRow = 0;
        boolean goingDown = false;
        for (char c : s.toCharArray()) {
            rows.get(currRow).append(c);
            if (currRow == 0 || currRow == numRows - 1) goingDown = !goingDown;
            currRow += goingDown ? 1 : -1;
        }
        StringBuilder ret = new StringBuilder();
        for (StringBuilder row : rows) ret.append(row);
        return ret.toString();
    }
}