class Solution {
    public String convert(String s, int R) {
        if(R == 1) return s;
        String res = "";
        for(int i = 0; i < R; i++){
            //第一行和最后一行,首项i, 公差2(R-1)
            if(i == 0 || i == R-1){
                for(int j = i; j < s.length(); j += 2*(R-1)){
                    res += s.charAt(j);
                }
            }else{
                //1. 首项为i, 公差为2(R-1)
                //2. 首项为2(R-1)-i, 公差为2(R-1)交替
                for(int k = i, m = 2*(R-1)-i; k < s.length() || m < s.length(); k += 2*(R-1), m += 2 *(R-1)){
                    if(k<s.length()) res += s.charAt(k);
                    if(m<s.length()) res += s.charAt(m);
                }
            }
        }
        return res;
    }
}