poj3921.Destroying the bus stations( 迭代加深搜索 && BFS )

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有一张 n 个点， m 条边的有向图，要你删除最少的点使得 1 -> n 的路径长度大于 k ，两点之间路径长均为 1 ，1 号点和 n 号点不能被删除。

因为数据规模比较小，直接搜索(迭代加深)，枚举删除的点数同时也作为深度，先 BFS 找到一条从 1 到 n 的路径，然后依次删除上面的点，分别进行 BFS 再次寻找路径，看能否大于k。

#include <cstdio>#include <cstring>using namespace std;const int MAX_N = 100;const int MAX_M = 4005;struct node{int v, next;}E[MAX_M];int head[MAX_N], top = 0;inline void add(int u, int v){E[++ top].v = v; E[top].next = head[u]; head[u] = top;}int n, m, k, id, to[MAX_N], q[MAX_N], arr[MAX_N][MAX_N];bool got = 0, vis[MAX_N];void init(){for(int i = 1; i <= m; i ++){int u, v; scanf("%d%d", &u, &v);add(u, v);}}void bfs(){memset(to, 0, sizeof(to));int begin = 0, end = 1;q[begin] = 1;while(begin < end){int x = q[begin];for(int i = head[x]; i; i = E[i].next)if(!vis[E[i].v] && to[E[i].v] == 0){to[E[i].v] = x; q[end] = E[i].v;end ++;if(E[i].v == n) break;}begin ++;if(to[n]) break;}}void dfs(int deep){if(got) return;bfs();if(to[n] == 0){ got = 1;  return; }int v = n, tot = -1;while(v){arr[deep][++ tot] = v;v = to[v];}if(tot > k){ got = 1; return; }if(deep > id) return;for(int i = 1; i < tot; i ++){vis[arr[deep][i]] = 1;dfs(deep + 1);vis[arr[deep][i]] = 0;}}void doit(){int ans = -1; got = 0;for(int i = 0; i <= n; i ++){id = i; memset(vis, 0, sizeof(vis));dfs(1);if(got) { ans = i; break; }}if(ans != -1)printf("%d\n", ans);else printf("%d\n", n);}int main(){freopen("1.in", "r", stdin);while(scanf("%d%d%d", &n, &m, &k) != EOF){if(n == 0 && m == 0 && k == 0) break;memset(head, 0, sizeof(head)); top = 0;init(); doit();}return 0;}

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