洛谷 P3183 [HAOI2016]食物链

题目传送门

解题思路:

跟另一个题差不多,就是本题单独一个生物不算一条食物链.

AC代码:

#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>

using namespace std;

int n,m,_in[100001],f[100001],_out[100001],ans;
vector<int> a[100001];
bool vis[100001];
queue<int> q;

inline void solve(int fa) {
    q.push(fa);
    f[fa] = 1;
    while(!q.empty()) {
        int u = q.front();
        q.pop();
        for(int i = 0;i < a[u].size(); i++) {
            f[a[u][i]] = f[a[u][i]] + f[u];
            _in[a[u][i]]--;
            if(_in[a[u][i]] == 0)
                q.push(a[u][i]);
        }
    }
}

int main() {
    scanf("%d%d",&n,&m);
    for(int i = 1;i <= m; i++) {
        int x,y;
        scanf("%d%d",&x,&y);
        _in[y]++;_out[x]++;
        a[x].push_back(y);
        vis[y] = 1;
    }
    for(int i = 1;i <= n; i++) 
        if(!vis[i])
            solve(i);
    for(int i = 1;i <= n; i++) 
        if(_out[i] == 0 && vis[i])
            ans += f[i];
    printf("%d",ans);
    return 0;
}