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".

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


 通过分析可知，位于第一行和最后一行的间隔为interval = numRows + numRows -2 = 2*(numRows - 1)

 现在分析其他行，我们发现，在其他行，假设行数为row(从0开始),第一个字符在原字符的row位置，而下一个字符在numsRows - row+ (numRows - row  - 2) = 2*(numRows - row - 1)，而下下个字符就在间隔interval处。

class Solution {
public:
    string convert(string s, int numRows) {
        if(numRows < 2)
            return s;
        int interval = (numRows - 1) <<1;
        string res;

        for(int i = 0; i < numRows; i++){
           int dis = (numRows - 1 - i)<<1;
           int j = i;
           while(j < s.size()){
               res += s[j];

               if(dis != 0 && dis != interval && (j + dis) < s.size()){
                   res += s[j + dis];
               }

               j += interval;
           }
        }
        return res;
    }
};