Zig Zag 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"