LeetCode 282: Expression Add Operation

class Solution {
    public List<String> addOperators(String num, int target) {
        List<String> result = new ArrayList<>();
        generateOperators(result, num, "", 0, 0, target, 0L);
        return result;
    }
    
    private void generateOperators(List<String> result, String num, String current, int index, long value, int target, long multi) {
        if (index == num.length()) {
            if (value == target) {
                result.add(current);
            }
            return;
        }
        
        for (int i = index; i < num.length(); i++) {
            if (i != index && num.charAt(index) == ‘0‘) {
                break;
            }
            
            long data = Long.parseLong(num.substring(index, i + 1));
            
            if (index == 0) {
                generateOperators(result, num, current + num.substring(index, i + 1), i + 1, value + data, target, data);
            } else {
                generateOperators(result, num, current + "+" + num.substring(index, i + 1), i + 1, value + data, target, data);
                generateOperators(result, num, current + "-" + num.substring(index, i + 1), i + 1, value - data, target, -data);
                generateOperators(result, num, current + "*" + num.substring(index, i + 1), i + 1, value - multi + multi * data, target, multi * data);
            }
        }
    }
}