From e7d1ce1e6ab8e209075509cda9513cc029644ecd Mon Sep 17 00:00:00 2001
From: rahulogre <rprahulpandey@yahoo.com>
Date: Sat, 27 Oct 2018 02:29:06 +0530
Subject: [PATCH] Shortest Path finding using BFS in an undirected graph

---
 shortest_path_BFS.cpp | 90 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 shortest_path_BFS.cpp

diff --git a/shortest_path_BFS.cpp b/shortest_path_BFS.cpp
new file mode 100644
index 0000000..3f4d390
--- /dev/null
+++ b/shortest_path_BFS.cpp
@@ -0,0 +1,90 @@
+    //finding shortest path in an undirected graph using BFS
+    #include <bits/stdc++.h>
+    using namespace std;
+     
+    #define pb push_back 
+    #define MAX 100001
+     
+    vector<int> G[MAX];
+    int parent[MAX];
+    bool visited[MAX];
+     
+     
+    void bfs(int s){
+     
+    	queue<int> q;
+    	q.push(s);
+    	visited[s] = true;
+     
+    	while(!q.empty()){
+    		int node = q.front();
+    		q.pop();
+     
+    		for(int i=0 ; i<G[node].size() ; i++){
+    			int curr = G[node][i];
+     
+    			if(!visited[curr]){
+    				parent[curr] = node;
+    				visited[curr] = true;
+    				q.push(curr);
+    			}
+     
+    		}
+    	}
+    } 
+     
+     
+    void construct_shortest_path(int s, int e){
+     
+    	vector<int> ans;
+    	for(int i=e ; i!=-1 ; ){
+    		ans.pb(i);
+    		i = parent[i];
+    	}
+     
+    	reverse(ans.begin(), ans.end());
+     
+    	int i;
+    	for(i=0 ; i<ans.size()-1 ; i++){
+    		cout<<ans[i]<<" -> ";
+    	}
+    	cout<<ans[i];
+     
+    }
+     
+     
+    void shortest_path(int s, int e){
+     
+    	bfs(s);
+    	construct_shortest_path(s,e);
+     
+    }
+     
+     
+     
+    int main() {
+     
+    	memset(visited, false, sizeof(visited));
+    	memset(parent, -1, sizeof(parent));
+     
+    	int nodes, edges;
+    	cin>>nodes>>edges;
+     
+    	for(int i=0; i <edges ; i++){
+    		int u,v;
+    		cin>>u>>v;
+     
+    		G[u].pb(v);
+    		G[v].pb(u);
+     
+    	}
+     
+    	int start,end;
+    	cin>>start>>end;
+     
+    	shortest_path(start,end);
+     
+    	return 0;
+    }
+     
+