Solution.cpp

class BinaryIndexedTree {
private:
    int n;
    vector<int> c;

public:
    BinaryIndexedTree(int n)
        : n(n)
        , c(n + 1) {}

    void update(int x, int delta) {
        for (; x <= n; x += x & -x) {
            c[x] += delta;
        }
    }

    int query(int x) {
        int s = 0;
        for (; x > 0; x -= x & -x) {
            s += c[x];
        }
        return s;
    }
};

class Solution {
public:
    int getPermutationIndex(vector<int>& perm) {
        const int mod = 1e9 + 7;
        using ll = long long;
        ll ans = 0;
        int n = perm.size();
        BinaryIndexedTree tree(n + 1);
        ll f[n];
        f[0] = 1;
        for (int i = 1; i < n; ++i) {
            f[i] = f[i - 1] * i % mod;
        }
        for (int i = 0; i < n; ++i) {
            int cnt = perm[i] - 1 - tree.query(perm[i]);
            ans += cnt * f[n - i - 1] % mod;
            tree.update(perm[i], 1);
        }
        return ans % mod;
    }
};