HDU-4255 BFS 最短路

题意：蛇形填数，然后素数处是障碍,给你起点终点，求步数；

思路：其实就是bfs,关键是将数字转换成位置比较难；

bfs其实比较简单，就是固定的思路，固定的步骤；

模板：

 const int dir[][] = {{-, }, {, }, {, }, {, -}};
 int vis[maxn], d[maxn];
 bool is_ok(int x, int y)///坐标是否合格，按照题意来进行
 {
     if(x< || y<||x>n||y>n)
         return false;
     return true;
 }
 int dfs(Node st,Node ed)///起点终点
 {
     queue<Node> q;
     q.push(st);///压进起点
     memset(vis,,sisteof(vis));
     memset(d,,sizeof(d));
     int ss = a[st.x][st.y];
     d[ss] = ;
     int edd = a[ed.x][ed.y];
     while(!q.empty())
     {
         Node c = q.front(),v;
         q.pop();
         int stt = a[c.x][c.y];
         if(stt == edd)///先判断是否到达终点
             return d[stt];
         repu(i,,)
         {
             v.x = c.x + dir[i][];
             v.y = c.y + dir[i][];
             if(is_ok(v.x,v.y))///坐标是否合格
             {
                 int sd = a[v.x][v.y];
                 if(!vis[sd])///符合所有条件后
                 {
                     q.push(v);///压进
                     vis[sd] = ;///V过
                     d[sd] = d[stt] + ;///步数为之前的+1
                 }
             }
             else
                 continue;
         }
     }
     return -;
 }

该题代码

 #include <iostream>
 #include <cstdio>
 #include <cmath>
 #include <cstring>
 #include <cstdlib>
 #include <string>
 #include <vector>
 #include <algorithm>
 #include<queue>
 #include <set>
 #define repu(i,a,b) for(int i=a;i<b;i++)
 using namespace std;
 #define N 1000010
 #define ll long long
 #define _cle(m, a) memset(m, a, sizeof(m))
 const int maxn = ;
 const int mn = ;
 int tot;
 int a[][];
 struct Node
 {
     int x, y;
 } nodes[maxn];
 void init()
 {
     memset(a, , sizeof(a));
     a[mn][mn] = ;
     tot = ;
     nodes[].x = mn;
     nodes[].y = mn;
     int cur = ;
     int i = mn, j = mn+;
     while(tot <= maxn)
     {
         int t;
         t = ;
         i++;
         while(t < cur*)
         {
             a[--i][j] = ++tot;
             nodes[tot].x = i;
             nodes[tot].y = j;
             t++;
         }
         t = ;
         while(t < cur*)
         {
             a[i][--j] = ++tot;
             nodes[tot].x = i;
             nodes[tot].y = j;
             t++;
         }
         t = ;
         while(t < cur*)
         {
             a[++i][j] = ++tot;
             nodes[tot].x = i;
             nodes[tot].y = j;
             t++;
         }
         t = ;
         while(t < cur*)
         {
             a[i][++j] = ++tot;
             nodes[tot].x = i;
             nodes[tot].y = j;
             t++;
         }
         ++j;
         cur++;
     }
 }
 int prime[maxn];
 void is_prime()
 {
     memset(prime, , sizeof(prime));
     int m = sqrt(maxn+0.5);
     prime[] = prime[] = ;
     for(int i = ; i <= m; i++)
     {
         if(!prime[i])
         {
             for(int j = i*i; j <= maxn; j+=i)
             {
                 prime[j] = ;
             }
         }
     }
 }
 
 const int dir[][] = {{-, }, {, }, {, }, {, -}};
 int vis[maxn], d[maxn];
 bool is_ok(int x, int y)
 {
     return x>= && y >=;
 }
 int bfs(Node z,Node b)
 {
     queue<Node> q;
     q.push(z);
     memset(vis,,sizeof(vis));
     memset(d,,sizeof(d));
     int ss = a[z.x][z.y];
     d[ss] = ;
     int ed = a[b.x][b.y];
     while(!q.empty())
     {
         Node c = q.front(),v;
         q.pop();
         int st = a[c.x][c.y];
         if(st == ed)
             return d[st];
         repu(i,,)
         {
             v.x = c.x + dir[i][];
             v.y = c.y + dir[i][];
             if(is_ok(v.x,v.y))
             {
                 int sd = a[v.x][v.y];
                 if(!vis[sd]&&prime[sd])
                 {
                     q.push(v);
                     vis[sd] = ;
                     d[sd] = d[st] + ;
                 }
             }
             else
                 continue;
         }
     }
     return -;
 }
 int main()
 {
     init();
     is_prime();
     int x, y, kase = ;
     while(~scanf("%d%d", &x, &y))
     {
         if(!prime[x] || !prime[y])
         {
             printf("Case %d: impossible\n", ++kase);
             continue;
         }
         int ans = bfs(nodes[x], nodes[y]);
         if(ans == -) printf("Case %d: impossible\n", ++kase);
         else printf("Case %d: %d\n", ++kase, ans);
     }
 
     return ;
 }