[CF] 402 E. Strictly Positive Matrix

#include<iostream>
#include<cstdio>

using namespace std;

const int MAXN=2048;

struct Edge{
  int next,to;
}e[MAXN*MAXN*2];
int ecnt,head[MAXN];
inline void add(int x,int y){
  e[++ecnt].to = y;
  e[ecnt].next = head[x];
  head[x] = ecnt;
}

inline int rd(){
  int ret=0,f=1;char c;
  while(c=getchar(),!isdigit(c))f=c==‘-‘?-1:1;
  while(isdigit(c))ret=ret*10+c-‘0‘,c=getchar();
  return ret*f;
}

int n;

int bl[MAXN],cnt;
int ins[MAXN],sta[MAXN],top;
int dfn[MAXN],low[MAXN],tot;
void tarjan(int x){
  dfn[x]=low[x]=++tot;ins[x]=1;sta[++top]=x;
  for(int i=head[x];i;i=e[i].next){
    int v=e[i].to;
    // cout<<x<<" "<<v<<endl<<endl;
    if(!dfn[v]){
      tarjan(v);
      low[x]=min(low[x],low[v]);
    }else if(ins[v]){
      low[x]=min(low[x],dfn[v]);
    }
  }
  if(dfn[x]==low[x]){
    int elm;cnt++;
    do{
        elm = sta[top--];
        ins[elm] = false;
        bl[elm] = cnt;
    }while(elm != x);
  }
}

int main(){
  n=rd();int x;
  for(int i=1;i<=n;i++)
    for(int j=1;j<=n;j++)
      if(rd()) add(i,j);
  for(int i=1;i<=n;i++) if(!dfn[i]) tarjan(i);
  if(cnt!=1) return puts("NO"),0;
  puts("YES");
  return 0;
}