#include <iostream>
#include<list>
#include<queue>
#include<unordered_map>
#include<set>
using namespace std;

void prepAdjList(unordered_map<int,list<int> &adjList , vector<pair<int,int>> &edges){
  for(int i=0;i<edges.size();i++){
    int u=edges[i].first;
    int v=edges[i].second;
    
    adjList[u].push_back(v);
    adjList[v].push_back(u);
  }
}

void bfs(unordered_map<int,list<int> &adjList, unordered_map<int, bool> &visited, vector<int>&ans, int node){
  
  queue<int> q;
  q.push(node);
  visited[node]=1;
  
  while(!q.isEmpty()){
    int frontend=q.front();
    q.pop();
    
    ans.push_back(frontend);
    
    for(auto i:adjList(frontend)){
      if (!visited[i]){
        q.push(i);
        visited[i]=1;
      }
    }
  }
}

vector<int> BFS(int vertex, vector<pair<int, int>> &edges){
  
  unordered_map<int, list<int>> adjList;
  unordered_map<int,bool> visited;
  vector<int> ans;
  prepAdjList(adjList,edges);
  
  //traverse all components of a graph
  for(int i=0;i<vertex;i++){
    if(!visited[i]){
      bfs(adjList,visited,ans,i);
    }
  }
  return ans;
}

int main() 
{
    int n;
    vector<pair<int, int>>edges;
    
    cin>>n;
    for(int i=0;i<n;i++){
      int u,v;
      cin>>u>>v;
      edges[i].first=u;
      edges[i].second=v;
    }
    vector<int> ans=BFS(n,edges);
    for (auto i:ans)cout<<i<<endl;
}
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Read inputs from stdin

OneCompiler's C++ online compiler supports stdin and users can give inputs to programs using the STDIN textbox under the I/O tab. Following is a sample program which takes name as input and print your name with hello.

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    string name;
    cout << "Enter name:";
    getline (cin, name);
    cout << "Hello " << name;
    return 0;
}

About C++

C++ is a widely used middle-level programming language.

  • Supports different platforms like Windows, various Linux flavours, MacOS etc
  • C++ supports OOPS concepts like Inheritance, Polymorphism, Encapsulation and Abstraction.
  • Case-sensitive
  • C++ is a compiler based language
  • C++ supports structured programming language
  • C++ provides alot of inbuilt functions and also supports dynamic memory allocation.
  • Like C, C++ also allows you to play with memory using Pointers.

Syntax help

Loops

1. If-Else:

When ever you want to perform a set of operations based on a condition If-Else is used.

if(conditional-expression) {
   //code
}
else {
   //code
}

You can also use if-else for nested Ifs and If-Else-If ladder when multiple conditions are to be performed on a single variable.

2. Switch:

Switch is an alternative to If-Else-If ladder.

switch(conditional-expression){    
case value1:    
 // code    
 break;  // optional  
case value2:    
 // code    
 break;  // optional  
......    
    
default:     
 code to be executed when all the above cases are not matched;    
}

3. For:

For loop is used to iterate a set of statements based on a condition.

for(Initialization; Condition; Increment/decrement){  
  //code  
}

4. While:

While is also used to iterate a set of statements based on a condition. Usually while is preferred when number of iterations are not known in advance.

while (condition) {  
// code 
}

5. Do-While:

Do-while is also used to iterate a set of statements based on a condition. It is mostly used when you need to execute the statements atleast once.

do {  
 // code 
} while (condition);

Functions

Function is a sub-routine which contains set of statements. Usually functions are written when multiple calls are required to same set of statements which increases re-usuability and modularity. Function gets run only when it is called.

How to declare a Function:

return_type function_name(parameters);

How to call a Function:

function_name (parameters)

How to define a Function:

return_type function_name(parameters) {  
 // code
}