Codeforces 1324D Pair of Topics

题目链接

这种类型题大多都是移项, (a_i + a_j > b_i + b_j (i < j)), 移项得, (a_i - b_i > b_j - a_j (i < j)), 再取反(b_i - a_i < a_j - b_j (i < j)), 则我们可以设新数组(C_i = b_i - a_i, D_i = a_i - b_i (i < j)), 按照顺序统计(D_j > C_i (i < j))的对数即可, 可以用树状数组 (离散化), 也可以直接二分查找

#include<bits/stdc++.h>
using namespace std;
#define ms(x,y) memset(x, y, sizeof(x))
#define lowbit(x) ((x)&(-x))
typedef long long LL;
typedef pair<int,int> pii;

const int maxn = 2e5+5;

LL C[maxn<<1], all[maxn<<1], a[maxn], b[maxn], c[maxn], d[maxn], cnt;

void add(int pos, int val) {
    for(; pos < (maxn<<1); pos += lowbit(pos))
        C[pos] += val;
}

int getsum(int pos) {
    int ret = 0;
    for(; pos; pos -= lowbit(pos))
        ret += C[pos];
    return ret;
}

void run_case() {
    int n; cin >> n;
    int allcnt = 0;
    for(int i = 1; i <= n; ++i) {
        cin >> a[i];
    }
    for(int i = 1; i <= n; ++i) {
        cin >> b[i];
    }
    for(int i = 1; i <= n; ++i) {
        d[i] = a[i] - b[i];
        c[i] = b[i] - a[i];
        all[++allcnt] = c[i];
        all[++allcnt] = d[i];
    }
    sort(all+1, all+1+allcnt);
    cnt = unique(all+1, all+1+allcnt) - all - 1;
    for(int i = 1; i <= n; ++i) {
        c[i] = lower_bound(all+1, all+1+cnt, c[i]) - all;
    }
    for(int i = 1; i <= n; ++i) {
        d[i] = lower_bound(all+1, all+1+cnt, d[i]) - all;
    }
    LL ans = 0;
    for(int i = 1; i <= n; ++i) {
        ans += getsum(d[i]-1);
        add(c[i], 1);
    }
    cout << ans;
}

int main() {
    ios::sync_with_stdio(false), cin.tie(0);
    cout.flags(ios::fixed);cout.precision(2);
    //int t; cin >> t;
    //while(t--)
    run_case();
    cout.flush();
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
#define ms(x,y) memset(x, y, sizeof(x))
#define lowbit(x) ((x)&(-x))
typedef long long LL;
typedef pair<int,int> pii;

const int maxn = 2e5+5;

vector<int> v;
int a[maxn], b[maxn], x;
LL ans = 0;

void run_case() {
    int n; cin >> n;
    for(int i = 1; i <= n; ++i) cin >> a[i];
    for(int i = 1; i <= n; ++i) cin >> b[i];
    for(int i = 1; i <= n; ++i) v.push_back(a[i] - b[i]);
    sort(v.begin(), v.end());
    for(int i = 0; i < n-1; ++i) {
        x = upper_bound(v.begin()+i+1, v.end(), -v[i]) - v.begin();
        ans += n - x;
    }
    cout << ans;
}

int main() {
    ios::sync_with_stdio(false), cin.tie(0);
    cout.flags(ios::fixed);cout.precision(2);
    //int t; cin >> t;
    //while(t--)
    run_case();
    cout.flush();
    return 0;
}