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题解：
莫名其妙的开瓶子？？？
/**@Author: STZG*@Language: C++*/#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include //#define DEBUG#define RI register intusing namespace std;typedef long long ll;//typedef __int128 lll;const int N=10000;const int MOD=1e9+7;const double PI = acos(-1.0);const double EXP = 1E-8;const int INF = 0x3f3f3f3f;int t,n,m,k,q;int a[N];int b[N];int c[N];int main(){#ifdef DEBUG freopen("input.in", "r", stdin); //freopen("output.out", "w", stdout);#endif scanf("%d",&n); int ans=0; for(int i = 1; i <=n; i++){ scanf("%d%d", &a[i],&b[i]); } for(int i = 1; i <=n; i++){ for(int j = 1; j <=n; j++){ if(b[i]==a[j]&&i!=j) c[j]=1; } } for(int i = 1; i <=n; i++){ ans+=c[i]; } cout << n-ans << endl; //cout << "Hello world!" << endl; return 0;}
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