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;
}
};