1.树状数组写起来感觉比归并好理解多了 2.求^和v的形状，求出任意一个后，可以采用将数组“倒过来”的方法，即将数组的大小关系重置一下 3.不仅是逆序对，正序对也很好求，改变遍历方向即可
代码:
const int maxn=2e6+7; const int INF=1e9; const ll INFF=1e18; int a[maxn],c[maxn],Lsmall[maxn],Rsmall[maxn],n; void add(int x,int y){for (;x<=n;x+=x&-x)c[x]+=y;} int ask(int x){int ans=0;for (;x;x-=x&-x)ans+=c[x];return ans;} ll solve() { ll cnt=0;mem(c,0); for (int i=n;i;i--) { Rsmall[i]=ask(a[i]-1); add(a[i],1); } mem(c,0); rep(i,1,n) { Lsmall[i]=ask(a[i]-1); add(a[i],1); cnt+=1ll*Lsmall[i]*Rsmall[i]; } return cnt; } int main() { ll ans1=0,ans2=0; scanf("%d",&n); rep(i,1,n)scanf("%d",&a[i]); ans1=solve(); rep(i,1,n)a[i]=n+1-a[i]; ans2=solve(); printf("%lld %lld\n",ans2,ans1); return 0; }