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".

解法：

先从上向下扫描，即0~nRows-1,，后扫斜对角线，即gap~1。

string convert(string s, int nRows){    if(nRows == 1) return s;    string res[nRows];    int i = 0, j, gap = nRows-2;    while(i < s.size())    {        for(j = 0; i < s.size() && j < nRows; ++j)            res[j] += s[i++];        for(j = gap; i < s.size() && j > 0; --j)            res[j] += s[i++];    }    string str = "";    for(i = 0; i < nRows; ++i)        str += res[i];    return str;}