6. ZigZag Conversion

------------------------------------------------------question--------------------------------------------------------
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 s, int numRows);
Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I

----------------------------------------------------solution---------------------------------------------------------
class Solution {
public:
    string convert(string s, int numRows) {
			string ans;
			if(numRows <=1) return s;
			int length = (numRows -1 )*2 ;  //one circle
			for(int i = 0; i < numRows && i < s.size(); ++i){
				for(int j = i ; j < s.size(); ){
					//cout<<s[j]<<" ";
					ans.push_back(s[j]);
					int interval= length-2*i;
					if(interval%length!=0  && j+interval < s.size()){
						ans.push_back(s[j+interval]);
						//cout<<s[j+interval]<<" ";
					}
					j+=length;	
				}
				//cout<<"||";
			}
			return ans;
    }
};

分析 自己读一遍