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   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".

class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows <= 1) return s;
        string res;
        int size = 2 * numRows - 2;
        for(int i = 0; i < numRows; ++ i)
        {
            for(int j = i; j < s.size(); j += size)
            {
                res += s[j];
                int tmp = j + size - 2 * i;
                if(i != 0 && i != numRows - 1 && tmp < s.size())
                    res += s[tmp];
            }
        }
        return res;
    }
};

参考：http://www.cnblogs.com/springfor/p/3889414.html