[构造] Codeforces 717E Bubble Cup 9 - Finals E. Paint it really, really dark gray

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这是一道递归构造的题

先构造完子树，然后可以通过走father来调整自己

因为1没有father 需要特判走一个儿子来调整

#include<cstdio>#include<cstdlib>#include<algorithm>using namespace std;inline char nc(){  static char buf[100000],*p1=buf,*p2=buf;  if (p1==p2){ p2=(p1=buf)+fread(buf,1,100000,stdin); if (p1==p2) return EOF; }  return *p1++;}inline void read(int &x){  char c=nc(),b=1;  for (;!(c>='0' && c<='9');c=nc()) if (c=='-') b=-1;  for (x=0;c>='0' && c<='9';x=x*10+c-'0',c=nc()); x*=b;}const int N=200005;struct edge{  int u,v,next;}G[N<<1];int head[N],inum;inline void add(int u,int v,int p){  G[p].u=u; G[p].v=v; G[p].next=head[u]; head[u]=p;}int n;int tag[N],clr[N];#define V G[p].vinline void dfs(int u,int fa){  tag[u]=clr[u];  for (int p=head[u];p;p=G[p].next)    if (V!=fa)      dfs(V,u),tag[u]|=tag[V];}inline void solve(int u,int fa){  if (!tag[u]) return;  printf("%d ",u); clr[u]^=1;  for (int p=head[u];p;p=G[p].next)    if (V!=fa)      solve(V,u);  if (fa){    if (clr[u])      printf("%d %d %d ",fa,u,fa),clr[u]^=1;    else      printf("%d ",fa),clr[fa]^=1;  }}int main(){  int iu,iv;  freopen("t.in","r",stdin);  freopen("t.out","w",stdout);  read(n);  for (int i=1;i<=n;i++) read(clr[i]),clr[i]=(clr[i]==-1)?1:0;  for (int i=1;i<n;i++){    read(iu); read(iv);    add(iu,iv,++inum); add(iv,iu,++inum);  }  if (clr[1]==1){    int flag=0;    for (int i=2;i<=n;i++)      flag|=clr[i];    if (!flag)      return printf("%d %d %d %d ",1,G[head[1]].v,1,G[head[1]].v),0;  }  clr[1]^=1;  dfs(1,0);  solve(1,0);  if (clr[1]){    iv=G[head[1]].v;    printf("%d %d %d ",iv,1,iv);  }  return 0;}

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