题目链接：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”.

这道题的要求是对字符串进行ZigZag转化。举个例子：

A     G     M
B   F H   L N
C E   I K   O
D     J     P

由ABCDEFGHIJKLMNOP转化成AGMBFHLNCEIKODJP。

理解了题意，就变得简单了。在转化的时候，可以按垂直方向生成字符串，然后按水平方向读取字符串。

时间复杂度：O(n)

空间复杂度：O(1)

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class Solution
{
public:
    string convert(string s, int nRows)
    {
        vector<string> v(nRows);
        for(int i = 0; i < s.size(); )
        {
            for(int j = 0; i < s.size() && j < nRows; ++i, ++j)
                v[j] += s[i];
            for(int j = nRows-2; i < s.size() && j > 0; ++i, --j)
                v[j] += s[i];
        }
        
        string res = "";
        for(int i = 0; i < nRows; ++i)
            res += v[i];
        return res;
    }
};