Consider a Bad Guy with skills [a,b,c]. The set of gunmen that can't beat him is a cuboid with opposite corners [1,1,1] and [a,b,c]. The union U of all these cuboids is exactly the set of gunmen that can't be heroes. Thus the answer is M3 minus the volume of U. 

We can compute the volume of U in O(MlogM) by sweeping in one direction and maintaining the intersection of the sweeping plane and U in some tree-like structure. 

不太明白这里为什么要用Tree-like Structure，我是用partial_sort把最大的几个先计算出来的，可是超时阿