6.Z字形变换

题目
将字符串 "PAYPALISHIRING" 以Z字形排列成给定的行数：

P A H N
A P L S I I G
Y I R
之后从左往右，逐行读取字符："PAHNAPLSIIGYIR"

实现一个将字符串进行指定行数变换的函数:

string convert(string s, int numRows);

示例 1:
输入: s = "PAYPALISHIRING", numRows = 3
输出: "PAHNAPLSIIGYIR"

示例 2:
输入: s = "PAYPALISHIRING", numRows = 4
输出: "PINALSIGYAHRPI"
解释:

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

思路
用min(numRows, int(s.size()))来定义Z形的行数；定义变量currentRows 用作当前行，当当前行触碰到最上行和最下行时，反向赋值。

/**第一次写代码将goingDown = !goingDown; 写成了goingDown ！= goingDown;，导致程序错误**/
        if (numRows == 1) return s;

        vector<string> newRows(min(numRows, int(s.size())));
        int currentRows = 0;
        bool goingDown = false;
        for(char eve : s)
        {
            newRows[currentRows] += eve;
            if (currentRows == 0 || currentRows == numRows - 1) goingDown = !goingDown;
            currentRows += goingDown ? 1 : -1;
        }

        string res;
        for (string newRow : newRows) res += newRow;
        return res;