ZigZag Conversion [LeetCode]

 

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".

Summary: just finds the index in string s of every rows.

    string convert(string s, int nRows) {
        string output;
        if(nRows <= 0)
            return output;
        if(nRows == 1 || nRows == s.size())
            return s;
            
        for(int i = 0; i < nRows; i ++) {
            if(i == 0 || i + 1 == nRows){
                int j = i;
                while(j < s.size()){
                    output += s[j];
                    j += nRows + nRows - 2;
                }
            }else{
                int j = 0 + i;
                while(j < s.size()){
                    output += s[j];
                    int next_idx = j + nRows - (i + 1) + nRows - (i + 1);
                    if(next_idx < s.size())
                        output += s[next_idx];
                    j += nRows + nRows - 2;
                }
            }                                                                                                                                                                                                                               
        }
        return output;
    }