# Print the lexicographically smallest BFS of the graph starting from 1

• Difficulty Level : Easy
• Last Updated : 06 Jul, 2021

Given a connected graph with N vertices and M edges. The task is to print the lexicographically smallest BFS traversal of the graph starting from 1.
Note: The vertices are numbered from 1 to N.
Examples:

```Input: N = 5, M = 5
Edges:
1 4
3 4
5 4
3 2
1 5
Output: 1 4 3 2 5
Start from 1, go to 4, then to 3 and then to 2 and to 5.

Input: N = 3, M = 2
Edges:
1 2
1 3
Output: 1 2 3 ```

Approach: Instead of doing a normal BFS traversal on the graph, we can use a priority queue(min heap) instead of a simple queue. When a node is visited add its adjacent nodes into the priority queue. Every time, we visit a new node, it will be the one with the smallest index in the priority queue. Print the nodes when every time we visit them starting from 1.
Below is the implementation of the above approach:

## CPP

 `// C++ program to print the lexcicographically``// smallest path starting from 1` `#include ``using` `namespace` `std;` `// Function to print the smallest lexicographically``// BFS path starting from 1``void` `printLexoSmall(vector<``int``> adj[], ``int` `n)``{``    ``// Visited array``    ``bool` `vis[n + 1];``    ``memset``(vis, 0, ``sizeof` `vis);` `    ``// Minimum Heap``    ``priority_queue<``int``, vector<``int``>, greater<``int``> > Q;` `    ``// First one visited``    ``vis[1] = ``true``;``    ``Q.push(1);` `    ``// Iterate till all nodes are visited``    ``while` `(!Q.empty()) {` `        ``// Get the top element``        ``int` `now = Q.top();` `        ``// Pop the element``        ``Q.pop();` `        ``// Print the current node``        ``cout << now << ``" "``;` `        ``// Find adjacent nodes``        ``for` `(``auto` `p : adj[now]) {` `            ``// If not visited``            ``if` `(!vis[p]) {` `                ``// Push``                ``Q.push(p);` `                ``// Mark as visited``                ``vis[p] = ``true``;``            ``}``        ``}``    ``}``}` `// Function to insert edges in the graph``void` `insertEdges(``int` `u, ``int` `v, vector<``int``> adj[])``{``    ``adj[u].push_back(v);``    ``adj[v].push_back(u);``}` `// Driver Code``int` `main()``{``    ``int` `n = 5, m = 5;``    ``vector<``int``> adj[n + 1];` `    ``// Insert edges``    ``insertEdges(1, 4, adj);``    ``insertEdges(3, 4, adj);``    ``insertEdges(5, 4, adj);``    ``insertEdges(3, 2, adj);``    ``insertEdges(1, 5, adj);` `    ``// Function call``    ``printLexoSmall(adj, n);` `    ``return` `0;``}`

## Java

 `// Java program to print the lexcicographically``// smallest path starting from 1``import` `java.util.*;``public` `class` `GFG``{``  ` `  ``// Function to print the smallest lexicographically``  ``// BFS path starting from 1``  ``static` `void` `printLexoSmall(Vector> adj, ``int` `n)``  ``{``    ``// Visited array``    ``boolean``[] vis = ``new` `boolean``[n + ``1``];` `    ``// Minimum Heap``    ``Vector Q = ``new` `Vector();` `    ``// First one visited``    ``vis[``1``] = ``true``;``    ``Q.add(``1``);` `    ``// Iterate till all nodes are visited``    ``while` `(Q.size() > ``0``) {` `      ``// Get the top element``      ``int` `now = Q.get(``0``);` `      ``// Pop the element``      ``Q.remove(``0``);` `      ``// Print the current node``      ``System.out.print(now + ``" "``);` `      ``// Find adjacent nodes``      ``for``(``int` `p : adj.get(now)) {` `        ``// If not visited``        ``if` `(!vis[p]) {` `          ``// Push``          ``Q.add(p);``          ``Collections.sort(Q);` `          ``// Mark as visited``          ``vis[p] = ``true``;``        ``}``      ``}``    ``}``  ``}` `  ``// Function to insert edges in the graph``  ``static` `void` `insertEdges(``int` `u, ``int` `v, Vector> adj)``  ``{``    ``adj.get(u).add(v);``    ``adj.get(v).add(u);``  ``}` `  ``// Driver code``  ``public` `static` `void` `main(String[] args) {``    ``int` `n = ``5``;``    ``Vector> adj = ``new` `Vector>();    ``    ``for``(``int` `i = ``0``; i < n + ``1``; i++)``    ``{``      ``adj.add(``new` `Vector());``    ``}` `    ``// Insert edges``    ``insertEdges(``1``, ``4``, adj);``    ``insertEdges(``3``, ``4``, adj);``    ``insertEdges(``5``, ``4``, adj);``    ``insertEdges(``3``, ``2``, adj);``    ``insertEdges(``1``, ``5``, adj);` `    ``// Function call``    ``printLexoSmall(adj, n);``  ``}``}` `// This code is contributed by divyeshrabadiya07.`

## Python3

 `# Python program to print the lexcicographically``# smallest path starting from 1` `# Function to print the smallest lexicographically``# BFS path starting from 1``def` `printLexoSmall(adj, n):``    ` `    ``# Visited array``    ``vis ``=` `[``False` `for` `i ``in` `range``(n ``+` `1``)]``    ` `    ``# Minimum Heap``    ``Q ``=` `[]``    ` `    ``# First one visited``    ``vis[``1``] ``=` `True``;``    ``Q.append(``1``)``    ` `    ``# Iterate till all nodes are visited``    ``while``(``len``(Q) !``=` `0``):``        ` `        ``# Get the top element``        ``now ``=` `Q[``0``]``        ` `        ``# Pop the element``        ``Q.pop(``0``)``        ` `        ``# Print the current node``        ``print``(now, end ``=` `" "``)``        ` `        ``# Find adjacent nodes``        ``for` `p ``in` `adj[now]:``            ` `            ``# If not visited``            ``if``(``not` `vis[p]):``                ` `                ``# Push``                ``Q.append(p)``                ``Q.sort()``                ` `                ``# Mark as visited``                ``vis[p] ``=` `True` `# Function to insert edges in the graph``def` `insertEdges(u, v, adj):``    ``adj[u].append(v)``    ``adj[v].append(u)` `# Driver code``n ``=` `5``m ``=` `5``adj ``=` `[[] ``for` `i ``in` `range``(n ``+` `1``)]` `# Insert edges``insertEdges(``1``, ``4``, adj)``insertEdges(``3``, ``4``, adj)``insertEdges(``5``, ``4``, adj)``insertEdges(``3``, ``2``, adj)``insertEdges(``1``, ``5``, adj)` `# Function call``printLexoSmall(adj, n)` `# This code is contributed by avanitrachhadiya2155`

## C#

 `// C# program to print the lexcicographically``// smallest path starting from 1``using` `System;``using` `System.Collections.Generic;``class` `GFG {``    ` `    ``// Function to print the smallest lexicographically``    ``// BFS path starting from 1``    ``static` `void` `printLexoSmall(List> adj, ``int` `n)``    ``{``        ``// Visited array``        ``bool``[] vis = ``new` `bool``[n + 1];``      ` `        ``// Minimum Heap``        ``List<``int``> Q = ``new` `List<``int``>();``      ` `        ``// First one visited``        ``vis[1] = ``true``;``        ``Q.Add(1);``      ` `        ``// Iterate till all nodes are visited``        ``while` `(Q.Count > 0) {``      ` `            ``// Get the top element``            ``int` `now = Q[0];``      ` `            ``// Pop the element``            ``Q.RemoveAt(0);``      ` `            ``// Print the current node``            ``Console.Write(now + ``" "``);``      ` `            ``// Find adjacent nodes``            ``foreach` `(``int` `p ``in` `adj[now]) {``      ` `                ``// If not visited``                ``if` `(!vis[p]) {``      ` `                    ``// Push``                    ``Q.Add(p);``                    ``Q.Sort();``      ` `                    ``// Mark as visited``                    ``vis[p] = ``true``;``                ``}``            ``}``        ``}``    ``}``    ` `    ``// Function to insert edges in the graph``    ``static` `void` `insertEdges(``int` `u, ``int` `v, List> adj)``    ``{``        ``adj[u].Add(v);``        ``adj[v].Add(u);``    ``}` `  ``// Driver code``  ``static` `void` `Main()``  ``{``    ``int` `n = 5;``    ``List> adj = ``new` `List>();    ``    ``for``(``int` `i = 0; i < n + 1; i++)``    ``{``        ``adj.Add(``new` `List<``int``>());``    ``}``  ` `    ``// Insert edges``    ``insertEdges(1, 4, adj);``    ``insertEdges(3, 4, adj);``    ``insertEdges(5, 4, adj);``    ``insertEdges(3, 2, adj);``    ``insertEdges(1, 5, adj);``  ` `    ``// Function call``    ``printLexoSmall(adj, n);``  ``}``}` `// This code is contributed by divyesh072019.`

## Javascript

 ``
Output:
`1 4 3 2 5`

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