LeetCode (Google) 679 _24Game

You have 4 cards each containing a number from 1 to 9. You need to judge whether they could operated through *, /, +, -, (, )to get the value of 24.

Example 1:

Input: [4, 1, 8, 7]
Output: True
Explanation: (8-4) * (7-1) = 24

Example 2:

Input: [1, 2, 1, 2]
Output: False

Note:

1. The division operator / represents real division, not integer division. For example, 4 / (1 - 2/3) = 12.
2. Every operation done is between two numbers. In particular, we cannot use - as a unary operator. For example, with [1, 1, 1, 1] as input, the expression -1 - 1 - 1 - 1 is not allowed.
3. You cannot concatenate numbers together. For example, if the input is [1, 2, 1, 2], we cannot write this as 12 + 12.

public class Game24 {  
    public boolean judgePoint24(int[] nums) {  
        ArrayList<Double> list = new ArrayList<>();  
        for (int v : nums) list.add((double) v);  
        return solve(list);  
    }  
  
    private boolean solve(ArrayList<Double> list) {  
        if (list.size() == 0)   return false;  
        if (list.size() == 1)   return Math.abs(list.get(0) - 24) < 1e-6;  
  
        for (int i = 0; i < list.size(); i++) {  
            for (int j = 0; j < list.size(); j++) {  
                if (i != j) {  
                    ArrayList<Double> num = new ArrayList<>();  
                    for (int k = 0; k < list.size(); k++) if (k!=i && k!=j){  
                        num.add(list.get(k));  
                    }  
                    for (int k = 0; k < 4; k++) {  
                        if (k < 2 && j > i) continue;;  
                        if (k == 0) num.add(list.get(i) + list.get(j));  
                        if (k == 1) num.add(list.get(i) * list.get(j));  
                        if (k == 2) num.add(list.get(i) - list.get(j));  
                        if (k == 3) {  
                            if (list.get(j) != 0) {  
                                num.add(list.get(i) / list.get(j));  
                            }else {  
                                continue;  
                            }  
                        }  
                        if (solve(num)) return true;  
                        num.remove(num.size() - 1);  
                    }  
                }  
            }  
        }  
        return false;  
    }  
}