二十四点问题

public class Solution {
    public boolean solution(List<Integer> list) {
        return helper(list, 0);
    }
    
    public boolean helper(List<Integer> list, int pre) {
        if (list.size() == 1) {
            return list.get(0) == 24;
        }
        int first = list.get(0);
        int second = list.get(1);
        List<Integer> new_list = new LinkedList<Integer>(list);
        new_list.remove(1);
        new_list.remove(0);
        new_list.add(0, first + second);
        if (helper(new_list, first)) {
            return true;
        }
        new_list.set(0, first - second);
        if (helper(new_list, first)) {
            return true;
        }
        int tmp = (first - pre) * second + pre;
        new_list.set(0, tmp);
        if (helper(new_list, pre)) {
            return true;
        }
        new_list.set(0, first * second);
        if (helper(new_list, first)) {
            return true;
        }
        return false;
    }
 
}