Submission #632094

#	Time	Username	Problem	Language	Result	Execution time	Memory
632094		TimDee	Thousands Islands (IOI22_islands)	C++17	55 / 100	40 ms	12844 KiB

islands

#include "islands.h"
#include <bits/stdc++.h>
using namespace std;

int n,m;
vector<int> u,v;

vector<int> vis(1e5+3,0);
vector<int> par(1e5+3,-1);
vector<vector<int>> adj(1e5+3,vector<int>(0));

int cycpt=-1;
vector<int> cyc;
int color=2;

void dfs(int u) {

  if (vis[u]==color) {cyc.push_back(u); cycpt=u; return;}
  else if (vis[u]) return;
  vis[u]=2;

  for (auto v:adj[u]) {

    dfs(v);
    if (cycpt!=-1) {
      cyc.push_back(u);
      return;
    }

  }
  vis[u]=1;

}

//#define false {}
//#define true {}
variant<bool, std::vector<int>>
//vector<int>
find_journey(int N, int M, vector<int> U, vector<int> V) {

  n=N,m=M,u=U,v=V;
  if (n==2) {
    vector<int> fv,sv;
    for (int x=0; x<m; ++x) if (u[x]==0) fv.push_back(x); else sv.push_back(x);
      int f=fv.size(), s=sv.size();
    if (f>=2 && s) {
      vector<int> r={fv[0],sv[0],fv[1],fv[0],sv[0],fv[1]};;
      return r;
    } 
    else return false;
  }

  vector<vector<int>>__p2(n,vector<int>(n,0));
  int _p2=1;
  for (int i=0; i<m; ++i) {
    __p2[u[i]][v[i]]=1;
  }
  for (int i=0; i<n; ++i) {
    for (int j=0; j<n; ++j) {
      if (i==j) continue;
      _p2&=__p2[i][j];
    }
  }

  if (_p2) {

    int a,b,c,d; 
    for (int i=0; i<m; ++i) {
      if (u[i]==0 && v[i]==1) a=i;
      if (u[i]==0 && v[i]==2) b=i;
      if (u[i]==1 && v[i]==0) c=i;
      if (u[i]==2 && v[i]==0) d=i;
    }
    vector<int> r = {a,c,b,d,c,a,d,b};
    return r;

  }

  int _p3=1, _p4=1;
  for (int i=0; i<m; i+=2) {
    _p3&=((u[i]==v[i+1])&&(v[i]==u[i+1]));
    _p4&=((u[i]==u[i+1])&&(v[i]==v[i+1]));
  }

  if (_p3) {

    vector<int> deg(1e5+3,0);
    for (int i=0; i<m; i+=2) {
      deg[u[i]]++;
      deg[v[i]]++;
      adj[u[i]].push_back(v[i]);
      adj[v[i]].push_back(u[i]);
    }

    int mx=0;
    for (auto x:deg) mx=max(mx,x);

    if (deg[0]>=2) {
      int a=-1,b=-1,c=-1,d=-1;
      int f=-1,s=-1;
      for (auto x:adj[0]) {
        if (f==-1) f=x;
        else {s=x; break;}
      }
      for (int i=0; i<m; ++i) {
        if (u[i]==0 && v[i]==f) a=i;
        if (u[i]==0 && v[i]==s) b=i;
        if (u[i]==f && v[i]==0) c=i;
        if (u[i]==s && v[i]==0) d=i;
      }
      vector<int> r = {a,c,b,d,c,a,d,b};
      return r;
    }
    if (mx<3) return false;

    vis[0]=1;
    queue<int> q; q.push(0);
    while (!q.empty()) {
      int x=q.front(); q.pop();
      if (deg[x]>=3) {

        vector<int> r;
        int a=-1,b=-1,c=-1,d=-1;
        vector<int> path;
        int p=x;
        while (p!=0) {
          for (int i=0; i<m; ++i) {
            if (u[i]==par[p] && v[i]==p) {
              path.push_back(i);
              break;
            }
          }
          p=par[p];
        }
        for (int i=path.size()-1; i>=0; --i) r.push_back(path[i]);

        int f=-1, s=-1;
        for (auto y:adj[x]) {
          if (y==par[x]) continue;
          if (f==-1) f=y;
          else {s=y; break;}
        }

        //cout<<f<<' '<<s<<'\n';
        if (f!=s) for (int i=0; i<m; ++i) {
          if (u[i]==x && v[i]==f) a=i;
          if (u[i]==x && v[i]==s) b=i;
          if (u[i]==f && v[i]==x) c=i;
          if (u[i]==s && v[i]==x) d=i;
        } else for (int i=0; i<m; ++i) {
          if (u[i]==x && v[i]==f && a==-1) a=i;
          if (u[i]==x && v[i]==s && a!=-1) b=i;
          if (u[i]==f && v[i]==x && c==-1) c=i;
          if (u[i]==s && v[i]==x && c!=-1) d=i;
        }
        vector<int> paiu={a,c,b,d,c,a,d,b};
        for (auto x:paiu) r.push_back(x);
        for (int i=0; i<path.size(); ++i) r.push_back(path[i]);
        return r;

      }
      for (auto y:adj[x]) {
        if (!vis[y]) {
          q.push(y);
          vis[y]=1;
          par[y]=x;
        }
      }
    }

    return false;

  }

  if (_p4) {

    for (int i=0; i<m; i+=2) {
      adj[u[i]].push_back(v[i]);
    }

    dfs(0); //cout<<cycpt;

    if (cycpt==-1) return false;
    
    vector<int> r;
    vector<int> path;
    //for (auto x:cyc) cout<<x<<' '; cout<<'\n';
    for (int i=cyc.size()-1; i; --i) {
      for (int j=0; j<m; ++j) {
        if (v[j]==cyc[i-1] && u[j]==cyc[i]) { path.push_back(j); break; }
      }
    }
    int save=-1;
    for (auto x:path) r.push_back(x);
    for (int i=cyc.size()-1; i; --i) {
      if (cyc[i]!=cycpt) continue;
      for (int j=0; j<m; ++j) {
        if (v[j]==cyc[i-1] && u[j]==cyc[i]) { save=j; r.push_back(j+1); r.push_back(j); break; }
      }
      break;
    }
    reverse(path.begin(),path.end());
    for (auto x:path) {if (x!=save) r.push_back(x); else r.push_back(x+1); }

    return r;

  }

}

Compilation message (stderr)

islands.cpp: In function 'std::variant<bool, std::vector<int, std::allocator<int> > > find_journey(int, int, std::vector<int>, std::vector<int>)':
islands.cpp:158:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
  158 |         for (int i=0; i<path.size(); ++i) r.push_back(path[i]);
      |                       ~^~~~~~~~~~~~
islands.cpp:53:45: warning: control reaches end of non-void function [-Wreturn-type]
   53 |   vector<vector<int>>__p2(n,vector<int>(n,0));
      |                                             ^
islands.cpp:74:37: warning: 'd' may be used uninitialized in this function [-Wmaybe-uninitialized]
   74 |     vector<int> r = {a,c,b,d,c,a,d,b};
      |                                     ^
islands.cpp:74:37: warning: 'c' may be used uninitialized in this function [-Wmaybe-uninitialized]
islands.cpp:74:37: warning: 'b' may be used uninitialized in this function [-Wmaybe-uninitialized]
islands.cpp:74:37: warning: 'a' may be used uninitialized in this function [-Wmaybe-uninitialized]

• Subtask 1: 5 / 5
• Subtask 2: 5 / 5
• Subtask 3: 21 / 21
• Subtask 4: 24 / 24
• Subtask 5: 0 / 45