LeetCode-ZigZag Conversion

原题：

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)

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”.

理解zigzag的含义：
从上至下，再从下至上走之字形状，分析规律：numRows=5，第一行间隔8=5*2-2，第二行间隔：8-2,2
第三行：8-4,4，以此类推。

class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows==1)return s;
        int n=s.size();
        int interval=numRows*2-2;
        int i,k=0;
        string res(n,' ');
        for(i=0;i<n;i+=interval){
            res[k++]=s[i];
        }
        for(i=1;i<numRows-1;i++){
            int inter=2*i;
            for(int j=i;j<n;j+=inter){
                res[k++]=s[j];
                inter=interval-inter;
            }
        }
        for(i=numRows-1;i<n;i+=interval){
            res[k++]=s[i];
        }
        return res;
    }
};