Leetcode 6. ZigZag Conversion

    public String convert(String s, int n) {
        
        //corner case
        if(s == null || "".equals(s) || n <= 0){
            return "";
        }
        if(n == 1){
            return s;
        }
        
        //two for loop, read 
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i < n; i++){
            for(int j = i; j < s.length(); j = j + 2*n - 2){
                sb.append(s.charAt(j));
                if(i != 0 && i != n - 1){
                    int temp = (j + 2*n -2) - 2*i;//-2 * i 
                    if(temp < s.length()){
                        sb.append(s.charAt(temp));
                    }
                }
            }
        }
        
        return sb.toString();
    }