[search] connectivity model of DFS + search order

---

AcWing 1112. maze  

 

sample input ：

2
3
.##
..#
#..
0 0 2 2
5
.....
###.#
..#..
###..
...#.
0 0 4 0

sample output :

YES
NO

---

#include <iostream>
#include <algorithm>
#include <cstring>

using namespace std;

const int N = 110;

int n;
char g[N][N];
bool st[N][N];
int xa, ya, xb, yb;

int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

bool dfs(int x, int y)
{
    if(g[x][y] == '#') return false;
    if(x == xb && y == yb) return true;
    
    st[x][y] = true;
    for(int i = 0; i < 4; i ++ )
    {
        int a = x + dx[i], b = y + dy[i];
        if(a < 0 || a >= n || b < 0 || b >= n) continue;
        if(st[a][b]) continue;
        if(g[a][b] == '#') continue;
        
        if(dfs(a, b)) return true;
    }
    
    return false;
}

int main()
{
    int T; cin >> T;
    while(T -- )
    {
        cin >> n;
        for(int i = 0; i < n; i ++ ) cin >> g[i];
        memset(st, 0, sizeof st);
        
        cin >> xa >> ya >> xb >> yb;
        
        if(dfs(xa, ya)) puts("YES");
        else puts("NO");
    }
    
    return 0;
}

---

AcWing 1113. Red and black  

 

sample input ：

6 9 
....#. 
.....# 
...... 
...... 
...... 
...... 
...... 
#@...# 
.#..#. 
0 0

sample output ：

45

---

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 25;

int n, m;
char g[N][N];
bool st[N][N];

int dx[] = {1, 0, -1, 0}, dy[] = {0, 1, 0, -1};

int dfs(int x, int y)
{
    int cnt = 1;
    
    st[x][y] = true;
    for(int i = 0; i < 4; i ++ )
    {
        int a = x + dx[i], b = y + dy[i];
        if(a < 0 || a >= n || b < 0 || b >= m) continue;
        if(g[a][b] != '.') continue;
        if(st[a][b]) continue;
        
        cnt += dfs(a, b);
    }
    return cnt;
}

int main()
{
    while(cin >> m >> n, m || n)
    {
        for(int i = 0; i < n; i ++ ) cin >> g[i];
        
        int x, y;
        for(int i = 0; i < n; i ++ )
            for(int j = 0; j < m; j ++ )
                if(g[i][j] == '@')
                {
                    x = i;
                    y = j;
                }
        memset(st, 0, sizeof st);
        cout << dfs(x, y) << endl;
    }
    
    return 0;
}

---

AcWing 1116. Horse walking day  

sample input ：

1
5 4 0 0

sample output ：

32

---

 

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 10;

int n, m;
bool st[N][N];
int ans;
int dx[8] = {-2, -1, 1, 2, 2, 1, -1, -2};
int dy[8] = {1, 2, 2, 1, -1, -2, -2, -1};

void dfs(int x, int y, int cnt)
{
    if (cnt == n * m)
    {
        ans ++ ;
        return;
    }
    st[x][y] = true;

    for (int i = 0; i < 8; i ++ )
    {
        int a = x + dx[i], b = y + dy[i];
        if (a < 0 || a >= n || b < 0 || b >= m) continue;
        if (st[a][b]) continue;
        dfs(a, b, cnt + 1);
    }

    st[x][y] = false;
}

int main()
{
    int T;
    scanf("%d", &T);
    while (T -- )
    {
        int x, y;
        scanf("%d%d%d%d", &n, &m, &x, &y);

        memset(st, 0, sizeof st);
        ans = 0;
        dfs(x, y, 1);

        printf("%d\n", ans);
    }

    return 0;
}

---

 AcWing 1117. The word chain

sample input ：

5
at
touch
cheat
choose
tact
a

sample output ：

23

Tips

Connected “ dragon ” by atoucheatactactouchoose.

---

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 21;

int n;
string word[N];
int g[N][N];
int used[N];
int ans;

void dfs(string dragon, int last)
{
    ans = max((int)dragon.size(), ans);

    used[last] ++ ;

    for (int i = 0; i < n; i ++ )
        if (g[last][i] && used[i] < 2)
            dfs(dragon + word[i].substr(g[last][i]), i);

    used[last] -- ;
}

int main()
{
    cin >> n;
    for (int i = 0; i < n; i ++ ) cin >> word[i];
    char start;
    cin >> start;

    for (int i = 0; i < n; i ++ )
        for (int j = 0; j < n; j ++ )
        {
            string a = word[i], b = word[j];
            for (int k = 1; k < min(a.size(), b.size()); k ++ )
                if (a.substr(a.size() - k, k) == b.substr(0, k))
                {
                    g[i][j] = k;
                    break;
                }
        }

    for (int i = 0; i < n; i ++ )
        if (word[i][0] == start)
            dfs(word[i], i);

    cout << ans << endl;

    return 0;
}

---

AcWing 1118. Divided into coprime groups   

sample input ：

6
14 20 33 117 143 175

sample output ：

3

---

1. Add a number to the last group
2. Open a new group  

according to Order of Subscripts successively Add all the numbers to the group in

---

 

#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int N = 10;

int n;
int p[N];
int group[N][N];
bool st[N];
int ans = N;

int gcd(int a, int b)
{
    return b ? gcd(b, a % b) : a;
}

bool check(int group[], int gc, int i)
{
    for (int j = 0; j < gc; j ++ )
        if (gcd(p[group[j]], p[i]) > 1)
            return false;
    return true;
}

void dfs(int g, int gc, int tc, int start)
{
    if (g >= ans) return;
    if (tc == n) ans = g;

    bool flag = true;
    for (int i = start; i < n; i ++ )
        if (!st[i] && check(group[g], gc, i))
        {
            st[i] = true;
            group[g][gc] = i;
            dfs(g, gc + 1, tc + 1, i + 1);
            st[i] = false;

            flag = false;
        }

    if (flag) dfs(g + 1, 0, tc, 0);
}

int main()
{
    cin >> n;
    for (int i = 0; i < n; i ++ ) cin >> p[i];

    dfs(1, 0, 0, 0);

    cout << ans << endl;

    return 0;
}