Codeforces 501C Misha and Forest 拓扑序

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题意:n个点组成的森林,n<=6e4,给出n个点度数和s[v]:与v相邻点的异或值,求出森林的边 
若v的degree为1,则有边v->s[v],按照拓扑序遍历树,依次更新出边即可  

#include <bits/stdc++.h>using namespace std;typedef pair<int,int> ii;const int N=2e5+20;int n,d[N],s[N],vis[N];queue<int> q;vector<ii> ans;void Topo(){memset(vis,0,sizeof(vis));for(int i=0;i<n;i++){if(d[i]==1)q.push(i),vis[i]=1;}while(!q.empty()){int u=q.front();q.pop();//d[u]若为0 则u->s[u]的边已经添加 if(d[u]==0)continue;ans.push_back(ii(u,s[u]));d[s[u]]--;s[s[u]]^=u;//删除点u后,更新结点s[u] if(vis[s[u]]==0&&d[s[u]]==1)q.push(s[u]),vis[s[u]]=1;}}int main(){while(cin>>n){ans.clear(); for(int i=0;i<n;i++)scanf("%d%d",&d[i],&s[i]);Topo();cout<<ans.size()<<endl;for(int i=0;i<ans.size();i++)cout<<ans[i].first<<' '<<ans[i].second<<endl;}return 0;}