zoj 3781 Paint the Grid Reloaded（前向星+缩点+最短路）

题目链接：

zoj 3781

题意：

给出一张只有黑色和白色的图，每次可以改变一个联通块的所有颜色，求整张图颜色一样时的最少步骤。

题解：

将每个联通块缩点，然后对于每个点进行搜索，寻找到达最远点的距离，然后所有距离的最小值就是答案，可以用前向星算法存边。

深度理解链式前向星

AC代码

#include <iostream>#include <algorithm>#include <cstdio>#include <cstring>#include <queue>#define INF 1e6using namespace std;const int maxn = 50;int n,m;char mapp[maxn][maxn];int col[maxn][maxn];int figuer[2000][2000];int mov[4][2] = {0,1,0,-1,1,0,-1,0};struct node{int x,y;};struct edge{int to,next;}e[100000];int head[2000];int d[2000];int tot;void add(int u,int v){    e[++tot].to = v;    e[tot].next = head[u];    head[u] = tot;}void dfs(int x, int y, char color, int paint){    col[x][y] = paint;    for(int i = 0 ; i < 4; i++)    {        node a;        a.x = x + mov[i][0],a.y = y + mov[i][1];        if(a.x > 0 && a.x <= n && a.y > 0 && a.y <= m )        {            int v = col[a.x][a.y];            if(mapp[a.x-1][a.y-1] != color && v && !figuer[paint][v])            {                figuer[paint][v] = 1;                figuer[v][paint] = 1;                //cout << paint << " " << v <<endl;                add(paint,v);                add(v,paint);            }            else if(mapp[a.x-1][a.y-1] == color && !v)            {                col[a.x][a.y] = paint;                dfs(a.x,a.y,color,paint);            }        }    }}int bfs(int k){    int re = 0;    queue<int>q;    int x, y;    memset(d, 0, sizeof d);    q.push(k);    d[k] = 1;    while(!q.empty()){        x = q.front(); q.pop();        for(int i = head[x]; i; i = e[i].next){            y = e[i].to;            if (d[y]) continue;            q.push(y);            d[y] = d[x]+1;            //cout << d[y] <<endl;            re = max(re, d[y] - 1);        }    }    //cout << "re " <<re <<endl;    return re;}int main(){    int t;    scanf("%d",&t);    while(t--)    {        scanf("%d%d",&n,&m);        memset(col,0,sizeof(col));        memset(head,0,sizeof(head));        memset(figuer,0,sizeof(figuer));        tot = 0;        for(int i = 0;i<n; i++)        {            scanf("%s",mapp[i]);        }        int colNumber = 1;        for(int i = 1; i <= n; i++)        {            for(int j = 1; j <= m; j++)            {                if(col[i][j]==0)                {                    dfs(i,j,mapp[i-1][j-1],colNumber);                    colNumber ++;                    //cout << colNumber << endl;                }            }        }        /*for(int i = 1; i <= colNumber; i++)        {            for(int j = 1; j <= colNumber; j++)                cout << col[i][j] << " ";            cout << endl;        }*/        int ans = INF;        for(int i = 1; i <= colNumber-1; i++)        {            ans = min(ans,bfs(i));        }        printf("%d\n",ans);    }    return 0;}/*22 2OXOX3 3XOXOXOXOX*/