invnum.py

class fenwick_tree():
    n=1
    data=[0 for i in range(n)]
    def __init__(self,N):
        self.n=N
        self.data=[0 for i in range(N)]
    def add(self,p,x):
        assert 0<=p<self.n,"0<=p<n,p={0},n={1}".format(p,self.n)
        p+=1
        while(p<=self.n):
            self.data[p-1]+=x
            p+=p& -p
    def sum(self,l,r):
        assert (0<=l and l<=r and r<=self.n),"0<=l<=r<=n,l={0},r={1},n={2}".format(l,r,self.n)
        return self.sum0(r)-self.sum0(l)
    def sum0(self,r):
        s=0
        while(r>0):
            s+=self.data[r-1]
            r-=r&-r
        return s

def invnum(n,a):
    bit=fenwick_tree(n)
    res=0
    for j in range(n):
        res+=j-bit.sum0(a[j])
        bit.add(a[j],1)
    return res

def inv_num(A):
    S=[(A[i],i) for i in range(len(A))]
    S.sort()
    return invnum(len(A),[s[1] for s in S])