Codeforces 989D A Shade of Moonlight

A Shade of Moonlight

列列式子发现， 对于同一个云来说只有反向的云才能和它相交， 然后我们发现这个东西有单调性，然后二分就好啦。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long

using namespace std;

const int N = 1e5 + ;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + ;
const double eps = 1e-;
const double PI = acos(-);

int n, l, w;
vector<int> a;
vector<int> b;

bool check(int x1, int x2) {
    return abs(x1 + x2 + l) < 1LL * (x2 + l - x1) * w;
}

int main() {
    scanf("%d%d%d", &n, &l, &w);
    for(int i = ; i <= n; i++) {
        int x, v; scanf("%d%d", &x, &v);
        if(v == ) a.push_back(x);
        else b.push_back(x);
    }
    LL ans = ;
    sort(a.begin(), a.end());
    sort(b.begin(), b.end());
    for(auto& x2 : b) {
        int low = ;
        int high = lower_bound(a.begin(), a.end(), x2) - a.begin() - ;
        int p = -;
        while(low <= high) {
            int mid = (low + high) >> ;
            if(check(a[mid], x2)) p = mid, low = mid + ;
            else high = mid - ;
        }
        ans += p + ;
    }
    printf("%lld\n", ans);
    return ;
}

/*
*/