HDU 3313 Key Vertex (dfs + bfs)

HDU 3313 Key Vertex (dfs + bfs)
HDU 3313 Key Vertex

Question Link

Question: A directed acyclic graph, finding cut points between s and t

Idea: first, spfa finds a shortest path. If it does not exist, all n are cut points.
Then, each time we perform dfs from s, find the farthest point that can pass through the shortest path, and then this point is the cut point. Next time we start dfs at this point, continue iteration until we find t.

Code:

#include 
 
  #include 
  
   #include 
   
    #include 
    
     #include using namespace std;const int N = 100005;int n, m, s, t, fa[N], vis[N], mark[N], d[N];int first[N], vv[N * 3], next[N * 3], en;const int INF = 0x3f3f3f3f;void addedge(int a, int b) {vv[en] = b;next[en] = first[a];first[a] = en++;}bool bfs() {queue
     
       Q;for (int i = 0; i < n; i++) d[i] = INF;memset(vis, 0, sizeof(vis));Q.push(s);vis[s] = 1;d[s] = 0;while (!Q.empty()) {int u = Q.front();vis[u] = 0;Q.pop();for (int i = first[u]; i + 1; i = next[i]) {int v = vv[i];if (d[v] > d[u] + 1) {d[v] = d[u] + 1;fa[v] = u;if (!vis[v]) {Q.push(v);vis[v] = 1;}}}}if (d[t] >= INF) return false;int tmp = t;memset(mark, 0, sizeof(mark));while (tmp != s) {mark[tmp] = 1;tmp = fa[tmp];}mark[s] = mark[t] = 1;return true;}void dfs(int u) {for (int i = first[u]; i + 1; i = next[i]) {int v = vv[i];if (vis[v]) continue;vis[v] = 1;if (mark[v]) {if (d[v] > d[s])s = v;continue;}dfs(v);}}int main() {while (~scanf("%d%d", &n, &m)) {memset(first, -1, sizeof(first));en = 0;int u, v;while (m--) {scanf("%d%d", &u, &v);addedge(u, v);}scanf("%d%d", &s, &t);if (!bfs()) {printf("%d\n", n);continue;}int ans = 1;memset(vis, 0, sizeof(vis));while (s != t) {dfs(s);ans++;}printf("%d\n", ans);}return 0;}