树状数组+离散化

poj2299http://poj.org/problem?id=2299

用树状数组求逆序数，因为这里a[i]最大达1e9而且输入数据里有0，必须要离散化。注意int溢出要用long long。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<vector>
#include<stack>
#include<queue>
#define lson l, m, rt<<1
#define rson m+1, r, rt<<1|1
#define IO ios::sync_with_stdio(false);cin.tie(0);
#define INF 1e9
#define MAXN 500010
const int MOD=1e9+7;
typedef long long ll;
using namespace std;
ll n, c[MAXN];
typedef struct{
    int v, id1, id2;
}Node;
Node node[MAXN];
bool cmp(const Node a, const Node b)
{
    return a.v<b.v; 
}
bool cmp2(const Node a, const Node b)
{
    return a.id1<b.id1; 
}
int lowbit(int t)
{
    return t&(-t);
}
void add(int x, int y)
{
    for(int i = x; i <= n; i += lowbit(i)){
        c[i] += y;
    }
}
int getsum(int x)
{
    int s=0;
    for(int i = x; i > 0; i -= lowbit(i)){
        s += c[i];
    }
    return s;
}
int main()
{
    IO; 
    while(cin >> n, n){
        ll ans=0, x;
        memset(c, 0, sizeof(c));
        for(int i = 1; i <= n; i++){
            cin >> node[i].v;
            node[i].id1 = i;//id1打标签，用于后期恢复原顺序 
        } 
        sort(node+1, node+n+1, cmp);//对值升序排列 
        for(int i = 1; i <= n; i++){
            node[i].id2 = i;//id2打标签按值从小到大，重新命名 
        }
        sort(node+1, node+n+1, cmp2);//通过id1恢复原顺序 
        for(int i = 1; i <= n; i++){
            add(node[i].id2, 1);
            ans += i-getsum(node[i].id2);//getsum(x)表示前面<=x的个数 
        }
        cout << ans << endl;
    }
    return 0;
}