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".

思路：

最开始题意理解不清楚，后来看了下面这组图以后就明白了，找规律解决即可。

nRows = 2
0 2 4 6 8
1 3 5 7 9

nRows = 3
0   4   8     12    16
1 3 5 7 9  11 13 15
2   6   10    14

nRows = 4
0     6       12       18
1   5 7    11 13    17 19
2 4   8 10    14 16
3     9       15

代码：

    string convert(string s, int nRows) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.  
        if(nRows == 1)
            return s;  
        vector<string> tmp(nRows,"");
        int step = 0, cur = 0;
        int l = s.length();
        while(cur < l){
            if(step == 0){
                int i;
                for(i = 0; i < nRows && cur+i < l; i++)
                    tmp[i] += s[cur+i];
                if(i < nRows)
                    break;
                cur += nRows;
            }
            else{
                tmp[nRows-1-step] += s[cur];
                cur++;
            }
            step = (step+1)%(nRows-1);
        }
        string result = "";
        for(int i = 0; i < nRows; i++)
            result += tmp[i];
        return result;
    }