Elementary Math Gym

居然没有看出是二分图匹配。。
还是题目做的 太少勒

#include<bits/stdc++.h>#include<iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;#define sf scanf#define pf printf#define mem(a,b) memset(a,b,sizeof(a));#define rep(i,a,b) for(int i=(a);i<=(b);++i)#define MP make_pair#define ULL unsigned long long#define LL   long long#define inf 0x3f3f3f3f#define md ((ll+rr)>>1)#define ls (i<<1)#define rs (ls|1)#define eps 1e-5#define ree freopen("in.txt","r",stdin);#define bug pf("----------------");#define N 10030#define M 1000020#define INF 1e9int n;int a[N],b[N];LL c[N];LL san[N];int fst[N],nxt[M],e,vv[M];int match[N];int cnt;int vis[N];void add(int u,int v){    vv[e]=v;nxt[e]=fst[u];fst[u]=e++;}void init(){    mem(fst,-1);e=0;}bool dfs(int x){    for(int i=fst[x];~i;i=nxt[i]){        int v=vv[i];        if(vis[v])continue;        vis[v]=1;        if(match[v]==-1||dfs(match[v])){            match[v]=x;return 1;        }    }    return 0;}int haxi(LL x){    return lower_bound(san+1,san+1+cnt,x)-san;}int main(){    //ree    init();    sf("%d",&n);    cnt=0;    rep(i,1,n){        sf("%d%d",&a[i],&b[i]);        san[++cnt]=a[i]-b[i];        san[++cnt]=a[i]+b[i];        san[++cnt]=1LL*a[i]*b[i];    }    sort(san+1,san+1+cnt);    cnt=unique(san+1,san+1+cnt)-san-1;    for(int i=1;i<=n;++i){        add(i,haxi(a[i]+b[i]));        add(i,haxi(a[i]-b[i]));        add(i,haxi(1LL*a[i]*b[i]));    }    int ans=0;    mem(match,-1);    for(int i=1;i<=n;++i){        mem(vis,0);        if(dfs(i)){            ++ans;        }    }    if(ans==n){        for(int i=1;i<=cnt;++i){            if(match[i]==-1)continue;            c[match[i]]=i;        }        for(int i=1;i<=n;++i){            LL u=san[c[i]];            if(a[i]+b[i]==u)                pf("%d + %d = %lld\n",a[i],b[i],u);            else if(a[i]-b[i]==u)                pf("%d - %d = %lld\n",a[i],b[i],u);            else if(1LL*a[i]*b[i]==u)                pf("%d * %d = %lld\n",a[i],b[i],u);        }    }    else puts("impossible");}