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SMU Summer 2023 Contest Round 6

时间:2023-08-03 12:58:26浏览次数:42  
标签:Summer temp Contest int SMU ans pair fac mod

Problem - D. Number Of Permutations

传送门

容斥原理

思路:利用容斥,首先所有可能的排列肯定是fac[n],然后可能会有三种 bad 的情况:

①第一个元素的排列是非递减

②第二种是第二个元素的排列是非递减

③这两个可能出现的重叠情况,意思就是说同时导致①②成立

这个时候我们利用容斥的思想,用fac[n]-①-②+③即可

我们把所有的pair按照第一个元素优先排列的方式把所有的pair sort 一下( sort 对pair的排序方式是默认第一个元素优先的),这个时候我们就保证了所有pair的第一个元素组成的排列的肯定是一个不严格递增的排列

求③的时候需要注意的一点是,在已经按照第一个元素排完序之后,如果存在s[i+1].se<s[i].se,那么就表示不会有第三种情况发生因为s[i].fi<=s[i+1].fi所以如果按照第二个元素非降序排序的话,就会导致s[i+1].fi<s[i].fi,所以,如果出现这种情况则表明③=0

code

#include <bits/stdc++.h>

using namespace std;
#define int long long
typedef pair<int, int> pii;
const int mod = 998244353, N = 3e5 + 5;
pii s[N];
int fac[N];
map<pii, int> ab;
map<int, int> a, b;

signed main () {
    ios::sync_with_stdio (false);
    cin.tie (nullptr), cout.tie (nullptr);
    int n;
    cin >> n;
    fac[0] = 1;
    for (int i = 1; i <= n; ++i)
        fac[i] = fac[i - 1] * i % mod;
    for (int i = 1; i <= n; ++i) {
        int x, y;
        cin >> x >> y;
        a[x]++, b[y]++, ab[{x, y}]++;
        s[i] = make_pair (x, y);
    }
    sort (s + 1, s + n + 1);
    int ans = fac[n], temp = 1;
    for (auto i: a)
        temp = temp * fac[i.second] % mod;
    ans = (ans - temp + mod) % mod, temp = 1;
    for (auto i: b)
        temp = temp * fac[i.second] % mod;
    ans = (ans - temp + mod) % mod, temp = 1;
    for (auto i: ab)
        temp = temp * fac[i.second] % mod;
    for (int i = 1; i < n; ++i)
        if (s[i + 1].second < s[i].second) temp = 0;
    ans = (ans + temp) % mod;
    cout << ans << endl;
    return 0;
}

标签:Summer,temp,Contest,int,SMU,ans,pair,fac,mod
From: https://www.cnblogs.com/Lazyboyjdj/p/17603015.html

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