[LeetCode] 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 text, int nRows);

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

找到间隔相邻的规律，O(n)的时间可以完成

class Solution {
private:
    int step[2];
public:
    string convert(string s, int nRows) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function    
        int steps = 2 * nRows - 2;
        
        string ret(s);
        int retSize = 0;
        for(int i = 0; i < nRows; i++)
        {
            int num = nRows - (i + 1) + nRows - (i + 1) - 1;
            num++;
            step[0] = num;
            if (step[0] == 0)
                step[0] = steps;
            step[1] = steps - num;
            if (step[1] == 0)
                step[1] = steps;
                
            int j = i;
            int index = 0;
            while(j < s.size())
            {
                ret[retSize++] = s[j];
                if (j == j + step[index])
                    break;
                j = j + step[index];
                index = 1 - index;
            }
        }    
        
        return ret;
    }
};