不务正业-搜索

最新推荐文章于 2020-08-20 15:45:27 发布

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最新推荐文章于 2020-08-20 15:45:27 发布

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本文链接：https://blog.youkuaiyun.com/googler_offer/article/details/78117649

本文详细介绍了深度优先搜索（深搜）和广度优先搜索（广搜）两种算法，并通过具体实例进行讲解。包括如何解决迷宫问题和湖泊计数问题的深搜应用，以及如何寻找丢失的牛这一广搜经典案例。

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搜索分为

一、深搜

dfs和回溯是不同的概念

例题：

1792:迷宫

访问过就不能再访问了

然后注意起始点或者终结点为#的情况

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int maxn = 101;
char map[maxn][maxn];
int vis[maxn][maxn];
int ha,la,hb,lb;
int n;
int dir[4][2] = {1,0,0,1,-1,0,0,-1};
int flag;
void dfs(int x,int y)
{
    if(x==hb&&y==lb)
    {
        flag = 1;
        return;
    }
    for(int i=0;i<4;i++)
    {
        int dx = x+dir[i][0];
        int dy = y+dir[i][1];
        if(map[dx][dy]=='.'&&!vis[dx][dy]&&dx>=0&&dx<n&&dy>=0&&dy<n)
        {
            vis[dx][dy] = 1;
            dfs(dx,dy);
            //vis[dx][dy] = 0;
        }
    }
    return;
}
int main()
{
    int t;
    scanf("%d",&t);
    while(t--)
    {
        memset(vis,0,sizeof(vis));
        flag = 0;
        scanf("%d",&n);
        for(int i=0;i<n;i++)
        cin>>map[i];
        scanf("%d %d %d %d",&ha,&la,&hb,&lb);
        if(map[ha][la]=='#'||map[hb][lb]=='#')
        {
            cout<<"NO"<<endl;
            continue;
        }
        vis[ha][la] = 1;
        dfs(ha,la);
        if(flag) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}

1388:Lake Counting

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int maxn = 101;
char map[maxn][maxn];
int vis[maxn][maxn];
int ha,la,hb,lb;
int n,m;
int cnt;
int dir[8][2] = {1,0,0,1,-1,0,0,-1,1,1,-1,-1,1,-1,-1,1};
void dfs(int x,int y)
{
    for(int i=0;i<8;i++)
    {
        int dx = x+dir[i][0];
        int dy = y+dir[i][1];
        if(map[dx][dy]=='W'&&dx>=0&&dx<n&&dy>=0&&dy<m&&!vis[dx][dy])
        {
            //cnt++;
            vis[dx][dy] = 1;
            dfs(dx,dy);
            //vis[dx][dy] = 0;
        }
    }
    return;
}
int main()
{
    cnt = 0;
    scanf("%d %d",&n,&m);
    for(int i=0;i<n;i++)
    cin>>map[i];
    for(int i=0;i<n;i++)
    {
        for(int j=0;j<m;j++)
        {
            if(map[i][j]=='W'&&!vis[i][j])
            {
                cnt++;
                vis[i][j] = 1;
                dfs(i,j);
            }
        }
    }
    cout<<cnt<<endl;
    return 0;
}

二、广搜

模板题：

2971:抓住那头牛

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
int n,m;
const int N = 200100;
struct node
{
    int x;
    int step;
};
int vis[N];
queue<node> q;
void bfs()
{
    int Step;
    int X;
    node temp;
    while(!q.empty())
    {
        temp = q.front();
        q.pop();
        X = temp.x;
        Step = temp.step;
        if(X==m)
        {
            cout<<Step<<endl;
            return;
        }
        if(X>=1&&!vis[X-1])
        {
            node tmp;
            vis[X-1] = 1;
            tmp.x = X-1;
            tmp.step = Step+1;
            q.push(tmp);
        }
        if(X<=m&&!vis[X+1])
        {
            node tmp;
            vis[X+1] = 1;
            tmp.x = X+1;
            tmp.step = Step+1;
            q.push(tmp);
        }
        if(X<=m&&!vis[X*2])
        {
            node tmp;
            vis[X*2] = 1;
            tmp.x = X*2;
            tmp.step = Step+1;
            q.push(tmp);
        }
    }

}
int main()
{
    while(cin>>n>>m)
    {
        memset(vis,0,sizeof(vis));
        vis[n] = 1;
        node tmp;
        tmp.x = n;
        tmp.step = 0;
        q.push(tmp);
        bfs();
    }
    return 0;
}