dataStructure@ Find if there is a path between two vertices in a directed graph

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<limits>
 5 #include<vector>
 6 using namespace std;
 7 const int maxn = 4;
 8 struct edge{
 9     int to, cost;
10     edge(int t){
11         this->to = t; this->cost = 0;
12     }
13 };
14 void addEdge(vector<edge> &edgelist, vector<vector<int> > &G, int from, int to){
15     edgelist.push_back(edge(to));
16     G[from].push_back(edgelist.size()-1);
17 }
18 void addDoubleEdge(vector<edge> &edgelist, vector<vector<int> > &G, int from, int to){
19     addEdge(edgelist,G,from,to);
20     addEdge(edgelist,G,to,from);
21 }
22 bool isCyclic(vector<edge> edgelist, vector<vector<int> > G,vector<bool> vis, vector<bool> RecStack, int v){
23     for(int i=0;i<G[v].size();++i){
24         edge e = edgelist[G[v][i]];
25         if(RecStack[e.to]) return true;
26         if(!vis[e.to]){
27             vis[e.to] = true; RecStack[e.to] = true;
28             if(isCyclic(edgelist,G,vis,RecStack,e.to)) return true;
29             RecStack[e.to] = false;
30         }
31     }
32     return false;
33 }
34 void isCyclicUtil(vector<edge> edgelist, vector<vector<int> > G){// find all cycles.
35     vector<bool> vis(G.size());
36     vector<bool> RecStack(G.size());
37     for(int i=0;i<vis.size();++i) vis[i]=false;
38     for(int i=0;i<RecStack.size();++i) RecStack[i]=false;
39     
40     for(int i=0;i<G.size();++i){
41         if(!vis[i]){
42             vis[i] = true; RecStack[i] = true;
43             if(isCyclic(edgelist,G,vis,RecStack,i)){
44                 cout<<i<<" starts a cycle"<<endl; 
45             }
46             RecStack[i] = false;
47         }
48     }
49 }
50 bool dfs(vector<edge> edgelist, vector<vector<int> > G, vector<bool> vis, int from, int to){
51     if(from == to) return true;
52     for(int i=0;i<G[from].size();++i){
53         edge e = edgelist[G[from][i]];
54         if(e.to == to) return true;
55         if(!vis[e.to]){
56             vis[e.to] = true;
57             if(dfs(edgelist, G, vis, e.to, to)) return true;
58         }
59     }
60     return false;
61 }
62 void isReachable(vector<edge> edgelist, vector<vector<int> > G, int from, int to){
63     vector<bool> vis(G.size());
64     for(int i=0;i<vis.size();++i) vis[i] = false;
65     vis[from] = true;
66     if(dfs(edgelist, G, vis, from, to)) cout<<from<<" and "<<to<<" are reachable to each other"<<endl;
67     else cout<<from<<" and "<<to<<" are not reachable to each other"<<endl;
68 }
69 void buildMap(vector<edge> &edgelist, vector<vector<int> > &G){
70     addEdge(edgelist,G,0,1);
71     addEdge(edgelist,G,0,2);
72     addEdge(edgelist,G,2,0);
73     addEdge(edgelist,G,1,2);
74     addEdge(edgelist,G,2,3);
75     addEdge(edgelist,G,3,3);
76 }
77 int main(){
78     vector<edge> edgelist;
79     vector<vector<int> > G(maxn);
80     
81     buildMap(edgelist,G);
82     
83     isCyclicUtil(edgelist, G);
84     
85     isReachable(edgelist, G, 1, 1);
86     
87     return 0;
88 }