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".

   #define tr(c,it) for(decltype((c).begin()) (it) = (c).begin(); (it) != (c).end(); (it)++)  
   
   string convert(string s, int nRows) {  
     int n = s.length();  
     vector<string> v(nRows);  
     int j = 0, increase = 1;  
     if(nRows == 1)return s;  
     for (int i = 0; i < n; i++)  
     {  
       if(increase)  
       {  
         v[j++] += s[i];  
         if(j == nRows){j -= 2;increase = 0;}  
       }  
       else  
       {  
         v[j--] += s[i];  
         if(j < 0){j += 2;increase = 1;}  
       }  
     }  
     string res;  
     tr(v, it)  
     {  
       res += *it;  
     }  
     return res;  
   }