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".

核心思想：找出循环单元 如上面的例子 当nRows是时，原串的每个字符对应的行数为 1 2 3 2 1 2 3 2 1 2 3 2 1 2 ，很容易看出循环结构为 1 2 3 2 长度为：2*3-2

代码如下：

class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows==1||s=="")
            return s;
        string ss[numRows];//用来存储相应行号的串
        int flag;//用来存储原串字母对应行号
        for(int i=0;i<s.size();i++){
            flag=(i+1)%(2*numRows-2);
            if(flag>numRows)
                flag=2*numRows-flag;
            if(flag==0)
                flag=2;
            ss[flag-1]+=s[i];
        }
        string ans;
        for(int i=0;i<numRows;i++)
            ans+=ss[i];
        return ans;
    }
};