(并查集 or BFS+二分)HDU5652

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并查集:

#include<cstdio>
#define N 505
using namespace std;
struct node
{
    int x,y;
};
char map[N][N];
node p[N*N];
int turnx[4]={0,0,-1,1};
int turny[4]={-1,1,0,0};
int father[N*N];
int rank[N*N];
int find(int x)
{
    if(father[x]!=x)
    father[x]=find(father[x]);
    return father[x];
}
void connect(int a,int b)
{
    a=find(a);
    b=find(b);
    if(a!=b)
    {
        if(rank[a]>rank[b])
        father[b]=a;
        else if(rank[a]<rank[b])
        father[a]=b;
        else
        {
            father[a]=b;
            rank[b]++;
        }
    }
}
int main()
{
    int T;
    int i,j;
    int n,m,q;
    int china,india;
    int dx,dy;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d",&n,&m);
        for(i=1;i<=n;i++)
        scanf("%s",map[i]+1);
        scanf("%d",&q);
        for(i=1;i<=q;i++)
        {
            scanf("%d%d",&p[i].x,&p[i].y);
            ++p[i].x,++p[i].y;
            map[p[i].x][p[i].y]='1';
        }
        for(i=1;i<=n*m+2;i++)
        {
            father[i]=i;
            rank[i]=0;
        }
        china=n*m+1;
        india=n*m+2;
        for(i=1;i<=m;i++)
        {
            if(map[1][i]=='0')
            connect(i,china);
            if(map[n][i]=='0')
            connect((n-1)*m+i,india);
        }
        for(i=1;i<=n;i++)
        {
            for(j=1;j<=m;j++)
            {
                if(map[i][j]=='0')
                {
                    if(i<n&&map[i+1][j]=='0')
                    connect(i*m+j,(i-1)*m+j);
                    if(j<m&&map[i][j+1]=='0')
                    connect((i-1)*m+j+1,(i-1)*m+j);
                }
            }
        }
        if(find(china)==find(india))
        {
            printf("-1
");
            continue;
        }
        for(i=q;i>0;i--)
        {
            map[p[i].x][p[i].y]='0';
            if(p[i].x==1)
            connect(p[i].y,china);
            if(p[i].x==n)
            connect((n-1)*m+p[i].y,india);
            for(j=0;j<4;j++)
            {
                dx=p[i].x+turnx[j];
                dy=p[i].y+turny[j];
                if(dx>0&&dx<=n&&dy>=0&&dy<=m&&map[dx][dy]=='0')
                connect((dx-1)*m+dy,(p[i].x-1)*m+p[i].y);
            }
            if(find(china)==find(india))
            break;
        }
        printf("%d
",i);
    }
    return 0;
}

BFS+二分:

#include<cstdio>
#include<queue>
#include<cstring>
#include<stack>
using namespace std;
int n,m;
char map[510][510];
int visit[510][510];
int turnx[4]={1,-1,0,0};
int turny[4]={0,0,1,-1};
struct node
{
    int x;
    int y;
};
node u[300000];
int yes(int x,int y)
{
    if(x<0||y<0||x>n+1||y>=m||map[x][y]=='1')
    return 0;
    return 1;
}
queue<node> s;
int bfs()
{
    int i;
    node p,q;
    p.x=0;
    p.y=0;
    visit[p.x][p.y]=1;
    s.push(p);
    while(!s.empty())
    {
        p=s.front();
        s.pop();
        if(p.x==n+1&&p.y==0)
        {
            while(!s.empty())
            s.pop();
            return 1;
        }
        for(i=0;i<4;i++)
        {
            q=p;
            q.x+=turnx[i];
            q.y+=turny[i];
            if(!visit[q.x][q.y]&&yes(q.x,q.y))
            {
                visit[q.x][q.y]=1;
                s.push(q);
            }
        }
    }
    return 0;
}
int main()
{
    int T,Q,i,begin,end,mid;
    scanf("%d",&T);
    while(T--)
    {
        while(!s.empty())
        s.pop();
        scanf("%d%d",&n,&m);
        for(i=0;i<m;i++)
        {
            map[0][i]='0';
            map[n+1][i]='0';
        }
        memset(visit,0,sizeof(visit));
        for(i=1;i<=n;i++)
        scanf("%s",map[i]);
        scanf("%d",&Q);
        for(i=1;i<=Q;i++)
        {
            scanf("%d%d",&u[i].x,&u[i].y);
            ++u[i].x;
        }
        begin=1;
        end=Q;
        while(begin<=end)
        {
            mid=(begin+end)/2;
            memset(visit,0,sizeof(visit));
            for(i=begin;i<=mid;i++)
            map[u[i].x][u[i].y]='1';
            for(i=mid+1;i<=end;i++)
            map[u[i].x][u[i].y]='0';
            if(bfs()==1)
            begin=mid+1;
            else
            end=mid;
            if(begin==end)
            {
                mid=begin;
                break;
            }
        }
        printf("%d
",mid);
    }
    return 0;
}