【HDOJ】2492 Ping pong

线段树+离散化。

 #include <cstdio>
 #include <cstring>
 #include <cstdlib>

 #define MAXN 20005
 #define lson l, mid, rt<<1
 #define rson mid+1, r, rt<<1|1

 int buf[MAXN], bk[MAXN];
 int sum[MAXN<<], n;

 int comp(const void *a, const void *b) {
     return *(int *)a - *(int *)b;
 }

 void PushUP(int rt) {
     sum[rt] = sum[rt<<] + sum[rt<<|];
 }

 void build(int l, int r, int rt) {
     sum[rt] = ;
     if (l == r)
         return ;
     int mid = (l+r)>>;
     build(lson);
     build(rson);
 }

 int query(int ll, int rr , int l, int r, int rt) {
     if (ll<=l && r<=rr)
         return sum[rt];
     int mid = (l+r)>>;
     int ret = ;
     if (ll <= mid)
         ret += query(ll, rr, lson);
     if (rr > mid)
         ret += query(ll, rr, rson);
     return ret;
 }

 void update(int x, int l, int r, int rt) {
     if (l == r) {
         ++sum[rt];
         return ;
     }
     int mid = (l+r)>>;
     if (x <= mid)
         update(x, lson);
     else
         update(x, rson);
     PushUP(rt);
 }

 int getIndex(int x) {
     int l=, r=n, mid;

     while (l <= r) {
         mid = (l+r)>>;
         if (bk[mid] == x)
             return mid;
         if (x > bk[mid])
             l = mid + ;
         else
             r = mid - ;
     }
     return ;
 }

 int main() {
     int t;
     int i;
     int index;
     __int64 ll, lg, rl, rg;
     __int64 ans;

     scanf("%d", &t);
     while (t--) {
         scanf("%d", &n);
         for (i=; i<=n; ++i) {
             scanf("%d", &buf[i]);
             bk[i] = buf[i];
         }
         build(, n, );
         qsort(bk+, n, sizeof(int), comp);
         ans = ;
         for (i=; i<=n; ++i) {
             index = getIndex(buf[i]);
             ll = query(, index, , n, );
             lg = i--ll;
             rl = index--ll;
             rg = n-index-lg;
             ans += ll*rg + lg*rl;
             update(index, , n ,);
         }
         printf("%I64d\n", ans);
     }

     return ;
 }