2016 Summer Camp Training 2 H q frosh Week

The silly question of finding the number of reverse order.

Use a merge sort or tree array or line segment tree

Merge sort code:

# include<stdio.h># define MAXN 500100int a[maxn],b[maxn];long long ans;void merge (int l, int r) {int k, p, Q;    if (L = = r) return;    int mid= (l+r) >>1;    Merge (L,mid);    Merge (mid+1,r);    p = l; Q = mid+1; k = l;    while (P<=mid | | q<=r) {        if (P>mid | | (Q<=r && a[p]>a[q])) {            b[k++]=a[q++];            ans+=mid-p+1;        }        else             b[k++]=a[p++];    }    for (k=l; k<=r; k++) a[k] = b[k];    return;} int main () {    int n;    while (~SCANF ("%d", &n)) {ans = 0;        for (int i=0;i<n;i++)            scanf ("%d", a+i);        Merge (0,n-1);        printf ("%i64d\n", ans);    }    return 0;}

　　

2016 Summer Camp Training 2 H q frosh Week