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".

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

string convert(string s, int nRows){
　　if(nRows == 1) return s;
    string res = "";
    int distance = 2*nRows-2,length = s.length();
    for(int i = 0; i  < nRows; ++ i){
        for(int j = 0 ; distance*j+i < length; ++ j ){
            res+=s[distance*j+i];
            int next = distance*j + i + distance-2*i;
            if( (i!=0) && (i!=nRows-1) && next < length) res +=s[next];
        }
    }
    return res;
}

int main(){
    cout<<convert("PAYPALISHIRING", 3)<<endl;
}