[Leetcode] 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".

注意如果用push_back的话会超内存，因为每次push_back后如果空间不够则新空间会是原来的两倍，为了避免这种情况还是直接定义长度为len的字符串吧。题目根本就没说清楚嘛， 下面是n=4时的结果：

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

class Solution {
public:
    string convert(string s, int nRows) {
        int len = s.length();
        if (len <= nRows || nRows == 1) return s;
        int steps;
        string res;
        res.resize(len);
        int idx = 0;
        for (int i = 0; i < nRows; ++i) {
            if (i == nRows - 1) steps = 2 * (nRows - 1);
            else steps = 2 * (nRows - i - 1);
            
            if (i == 0 || i == nRows - 1) {
                for (int j = i; j < len; j += steps) {
                    res[idx] = s[j];
                    ++idx;
                }
            } else {
                bool flag = true;
                for (int j = i; j < len;) {
                    res[idx] = s[j];
                    ++idx;
                    if (flag) j += steps;
                    else j += (2 * (nRows - 1) - steps);
                    flag = !flag;
                }
            }
        }
        return res;
    }
};