6. ZigZag Conversion

1. Description

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).
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);

2. Example

Example 1

Input: s = “PAYPALISHIRING”, numRows = 3
Output: “PAHNAPLSIIGYIR”

Example 2

Input: s = “PAYPALISHIRING”, numRows = 4
Output: “PINALSIGYAHRPI”

Example 3

Input: s = “A”, numRows = 1
Output: “A”

3. Constraints

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

4. Solutions

String

n = s.size()
Time complexity: O(n)
Space complexity: O(1)

class Solution {
public:
    string convert(const string &s, int rows) {
        const int n = s.size();
        const int cycle = 2 * rows - 2;
        if (rows == 1 || rows >= s.size()) {
            return s;
        }

        string zigzag;
        zigzag.reserve(n);
        for (int row = 0; row < rows; ++row) {
            for (int i = row; i < n; i += cycle) {
                zigzag.push_back(s[i]);

                int diagonal = i + cycle - 2 * row;
                if (row != 0 && row != rows - 1 && diagonal < n) {
                    zigzag.push_back(s[diagonal]);
                }
            }
        }

        return zigzag;
    }
};