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   NA P L S I I GY   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".

class Solution {public:    string convert(string s, int nRows) {        string str;        int n = s.length();        int i = 0;        int row = 0;        int distance = 2*nRows-2;        if(nRows == 1)  return s;        while(row < nRows)        {            int m = row;            if((row == 0)||(row == nRows-1))            {                while(m < n)                {                    str += s[m];                    m += distance;                }            }            else            {                int dis = distance-2*row;                while(m < n)                {                    str +=s[m];                    m += dis;                    dis = distance- dis;                }            }            row++;        }        return str;    }};