Leetcode/679.24Game.h at master · bluepp/Leetcode · GitHub

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/*
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:
The division operator / represents real division, not integer division. For example, 4 / (1 - 2/3) = 12.
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.
You cannot concatenate numbers together. For example, if the input is [1, 2, 1, 2], we cannot write this as 12 + 12.
*/
    bool judgePoint24(vector<int>& nums) {

        bool ret = false;
        double eps = 0.001;

        vector<double> arr(nums.begin(), nums.end());
        helper(arr, eps, ret);

        return ret;
    }

    void helper(vector<double> nums, double eps, bool &ret) {
        if (ret) {
            return;
        }

        if (nums.size() == 1) {
            if (abs(nums[0]-24) < eps) {
                ret = true;
                return;
            }
        }

        for (int i = 0; i < nums.size(); i++) {
            for (int j = 0; j < i; j++) {
                double p = nums[i], q = nums[j];

                vector<double> t {p+q, p-q, q-p, p*q};

                if (p > eps) {
                    t.push_back(q/p);
                }

                if (q > eps) {
                    t.push_back(p/q);
                }

                nums.erase(nums.begin()+i);
                nums.erase(nums.begin()+j);

                for (double d : t) {
                    nums.push_back(t);
                    helper(nums, eps, ret);
                    nums.pop_back();
                }

                nums.insert(nums.begin()+j, q);
                nums.insert(nums.begin()+i, p);

            }
        }
    }