CF1042D Petya and Array

思路：

离散化+树状数组（或权值线段树）。bit数组一定要开够。和求逆序对数量很像。

实现：

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MAXN = 200005;
ll a[MAXN], b[MAXN], bit[2 * MAXN];
int n;
int lowbit(int x)
{
    return x & -x;
}
ll sum(int x)
{
    ll ans = 0;
    while (x)
    {
        ans += bit[x];
        x -= lowbit(x);
    }
    return ans;
}
void add(int i, ll x, int maxn)
{
    while (i <= maxn)
    {
        bit[i] += x;
        i += lowbit(i);
    }
}
int main()
{
    ll t;
    while (cin >> n >> t)
    {
        memset(b, 0, sizeof b);
        memset(bit, 0, sizeof bit);
        for (int i = 1; i <= n; i++) 
        {
            cin >> a[i];
            b[i] = b[i - 1] + a[i];
        }
        vector<ll> v;
        for (int i = 0; i <= n; i++)
        {
            v.push_back(b[i]);
            v.push_back(b[i] + t);
        }
        sort(v.begin(), v.end());
        v.erase(unique(v.begin(), v.end()), v.end());
        ll ans = 0;
        for (int i = n; i >= 1; i--)
        {
            int pos = lower_bound(v.begin(), v.end(), b[i]) - v.begin() + 1; 
            add(pos, 1, v.size());
            pos = lower_bound(v.begin(), v.end(), b[i - 1] + t) - v.begin() + 1;
            ans += sum(pos - 1);
        }
        cout << ans << endl;
    }
    return 0;
}