POJ – 2299 Ultra-QuickSort

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思路：
一道典型的树状数组求逆序对题，不详细写了，主要就是练习一下离散化.

代码：

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#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <string>
#include <algorithm>
#include <cmath>
#include <queue>
#include <vector>
#include <map>
#include <cctype>
#include <sstream>
#define INF 1e9
#define ll long long
#define ms(a,val) memset(a,val,sizeof(a))
#define lowbit(x) ((x)&(-x))

using namespace std;
int bit[500005],a[500005],b[500005],n;
int sum(int pos) {
    int i=pos,ans=0;
    while (i > 0) {
        ans += bit[i];
        i -= lowbit(i);
    }
    return ans;
}
void add(int pos, int x) {
    int i = pos;
    while (i <= 500004) {
        bit[i] += x;
        i += lowbit(i);
    }
}
int main() {
    ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
    while (cin>>n&&n) {
        ll ans= 0;
        ms(bit, 0);
        for (int i = 0;i < n;i++) {
            cin >> a[i];
            b[i] = a[i];
        }
        sort(a, a + n);
        int size = unique(a, a + n)-a;
        for (int i = 0;i < n;i++) {
            int index = lower_bound(a, a + size, b[i]) - a+1;
            add(index, 1);
            ans =ans+(ll)(i-sum(index)+1);
        }
        cout << ans << "\n";
    }
    return 0;
}