ZigZag Conversion @Leetcode

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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".

The key point of this problem is to find the pattern of index of the output array. Here we can divide the problem in two part, the first part is rows without Zag and the rows in the middle that have Zag. The total number of rows is nRows. The number of rows has Zag is nRows-2. So when we designing the loop in the algorithm, we can put a judging condition as if(i>0&&i<nRows-1) then do..., it means the rows we are working on are in the middle. Then we output the chars row by row. Let's define the new output string as string ret. First of all, let's consider the first row, the elements will be added in ret consequently are s[0], s[0+zigsize], s[0+2*zigsize]..until reach the end. Then set the counter i=1, it points to the second row. The first element in second row is s[1], the second element in second row is s[1+zigsize-2*1]. The gap between each two zag element in a row is zigsize. Once we figure out this regularity, we can design the algorithm based on this. 

Here is the link that well-solved this problem.
http://blog.unieagle.net/2012/11/08/leetcode%E9%A2%98%E7%9B%AE%EF%BC%9Azigzag-conversion/