Solution.ts

class BinaryIndexedTree {
    private n: number;
    private c: number[];

    constructor(n: number) {
        this.n = n;
        this.c = Array(n + 1).fill(0);
    }

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

    query(x: number): number {
        let s = 0;
        for (; x > 0; x -= x & -x) {
            s += this.c[x];
        }
        return s;
    }
}

function getPermutationIndex(perm: number[]): number {
    const mod = 1e9 + 7;
    const n = perm.length;
    const tree = new BinaryIndexedTree(n + 1);
    let ans = 0;
    const f: number[] = Array(n).fill(1);
    for (let i = 1; i < n; ++i) {
        f[i] = (f[i - 1] * i) % mod;
    }
    for (let i = 0; i < n; ++i) {
        const cnt = perm[i] - 1 - tree.query(perm[i]);
        ans = (ans + cnt * f[n - i - 1]) % mod;
        tree.update(perm[i], 1);
    }
    return ans % mod;
}