expression add operators

List<String> ans;
    public List<String> (String num, int target) {
        ans = new ArrayList<>();
        if (num == null || num.length() == 0)
            return ans;
        dfs(num, "", 0, 0, target, 0);
        return ans;

    }
    public void dfs(String num, String path, int begin, long sum, int target, long prev) {
        if (begin >= num.length()) {
            if (sum == target) {
                ans.add(path);
            }
            return;
        }
        for (int i = begin; i < num.length(); ++i) {
            
            if (i != begin && num.charAt(begin) == '0')
                break;
            long val = Long.parseLong(num.substring(begin, i + 1));
            if (begin == 0) {
                dfs(num, path + val, i + 1, 0 + val, target, val);
            } else {
                dfs(num, path + "+" + val, i + 1, sum + val, target, val);
                dfs(num, path + "-" + val, i + 1, sum - val, target, -val);
                dfs(num, path + "*" + val, i + 1, sum + val * prev - prev, target, val * prev);
            }
        }
    }