/// problem: https://leetcode.com/problems/partition-to-k-equal-sum-subsets/

int Solve(int mask, int sum)
{
    if(mask == 0) return 1;
    int ans = 0;
//    for(int sub = 1 ; sub <= mask ; sub++){
//        if((sub | mask) == mask && preCal[sub] == sum){
//            ans |= Solve(mask ^ sub , k-1 , sum);
//        }
//    }
    for(int sub = mask ; sub > 0 ; sub = (sub - 1)&mask){
        if(preCal[sub] == sum){
            ans |= Solve(mask ^ sub , sum);
        }
    }
    return ans;

}

/// state: O(2^n)
/// time: O(3^n) why??