leetcode[6] 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 numRows) {if (numRows != 1) {string ss = "";int l = s.length() / (numRows * 2 - 2);int num = numRows * 2 - 2;for (int i = 0; i < numRows; i++) {for (int j = 0; j <= l; j++) {if (j*num + i < s.length())ss = ss + s[j*num + i];if (i != 0 && i != numRows - 1) {if (j*num + num - i < s.length())ss = ss + s[j*num + num - i];}}}return ss;}else return s;}};