二十四点问题

判断四个数是否可能通过加减乘得到二十四， 可以使用括号，思路极其像Expression Add Operators， 区别是在处理乘号的时候，一种是没有括号的，和Expression Add Operators一样，一种是带有括号的，其实就是把乘号当作加号来计算

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;
}

}