679. 24 点游戏（回溯）

class Solution {
    public boolean judgePoint24(int[] nums) {
        double[] a = new double[]{nums[0],nums[1],nums[2],nums[3]};
        return find(a);
    }
    public boolean find(double[] a) {
        if(a.length == 1) return Math.abs(a[0] - 24) < 0.01;

        for(int i = 0; i < a.length; i++) {
            for(int j = i + 1; j < a.length; j++) {
                double[] b = new double[a.length-1];
                for(int k = 0, index = 0; k < a.length; k++) {
                    if(k != i && k != j) {
                        b[index++] = a[k];
                    }
                }
                for(double d : compute(a[i],a[j])) {
                    b[b.length-1] = d;
                    if(find(b)) return true;
                }
            }
        }
        return false;
    }

    public double[] compute(double a, double b) {
        return new double[]{a+b,a*b,a-b,b-a,a/b,b/a};
    }
}