Solution2.cpp

class Node {
public:
    int l;
    int r;
    int v;
};

class SegmentTree {
public:
    vector<Node*> tr;

    SegmentTree(int n) {
        tr.resize(4 * n);
        for (int i = 0; i < tr.size(); ++i) tr[i] = new Node();
        build(1, 1, n);
    }

    void build(int u, int l, int r) {
        tr[u]->l = l;
        tr[u]->r = r;
        if (l == r) return;
        int mid = (l + r) >> 1;
        build(u << 1, l, mid);
        build(u << 1 | 1, mid + 1, r);
    }

    void modify(int u, int x, int v) {
        if (tr[u]->l == x && tr[u]->r == x) {
            tr[u]->v += v;
            return;
        }
        int mid = (tr[u]->l + tr[u]->r) >> 1;
        if (x <= mid)
            modify(u << 1, x, v);
        else
            modify(u << 1 | 1, x, v);
        pushup(u);
    }

    void pushup(int u) {
        tr[u]->v = tr[u << 1]->v + tr[u << 1 | 1]->v;
    }

    int query(int u, int l, int r) {
        if (tr[u]->l >= l && tr[u]->r <= r) return tr[u]->v;
        int mid = (tr[u]->l + tr[u]->r) >> 1;
        int v = 0;
        if (l <= mid) v += query(u << 1, l, r);
        if (r > mid) v += query(u << 1 | 1, l, r);
        return v;
    }
};

class Solution {
public:
    long long goodTriplets(vector<int>& nums1, vector<int>& nums2) {
        int n = nums1.size();
        vector<int> pos(n);
        for (int i = 0; i < n; ++i) pos[nums2[i]] = i + 1;
        SegmentTree* tree = new SegmentTree(n);
        long long ans = 0;
        for (int& num : nums1) {
            int p = pos[num];
            int left = tree->query(1, 1, p);
            int right = n - p - (tree->query(1, 1, n) - tree->query(1, 1, p));
            ans += 1ll * left * right;
            tree->modify(1, p, 1);
        }
        return ans;
    }
};