0   4   8
1 3 5 7 9
2   6

#include <cmath>
#include <iostream>
#include <string>

class Solution {
    public:
        std::string convert(const std::string& s, const int numRows) {
            if (numRows == 1) { return s; }
            const int size = s.size();
            std::string converted = "";
            const int num  = numRows + (numRows - 2);
            int row = 0;
            int i   = 0;
            int sub = 0;
            bool useDiff = true;
            while (row < numRows) {
                converted += s[row];
                if (row == 0 || row == numRows - 1) {
                    i = row + num;
                    while (i < size) {
                        std::string str(1, s[i]);
                        converted.append(str);
                        i += num;
                    }
                } else {
                    int diff = std::abs(num - sub);
                    i = diff;
                    while (i < size && row + i < size) {
                        if (useDiff) {
                            std::string str(1, s[row + i]);
                            converted.append(str);
                            useDiff = false;
                            i += sub;
                        } else {
                            std::string str(1, s[row + i]);
                            converted.append(str);
                            useDiff = true;
                            i += diff;
                        }
                    }
                }
                useDiff = true;
                sub += 2;
                row++;
            }

            return converted;
        }
};

int main()
{
    Solution solution;
    std::string s = "0123456789";
    int numRows = 3;
    std::cout << s << 'n';
    s = solution.convert(s, numRows);
    std::cout << s << 'n';
}